diff --git a/dtool/src/dtoolbase/dtoolbase.cxx b/dtool/src/dtoolbase/dtoolbase.cxx
index 8c7c7e7eab..54494d06b5 100644
--- a/dtool/src/dtoolbase/dtoolbase.cxx
+++ b/dtool/src/dtoolbase/dtoolbase.cxx
@@ -148,4 +148,18 @@ void (*global_mark_pointer)(void *ptr, size_t size, ReferenceCount *ref_ptr) = &
 
 #else
 
-#endif
+#endif  // USE_MEMORY_*
+
+
+#if defined(HAVE_THREADS) && defined(SIMPLE_THREADS)
+
+static void
+default_thread_yield() {
+}
+static void
+default_thread_consider_yield() {
+}
+void (*global_thread_yield)() = default_thread_yield;
+void (*global_thread_consider_yield)() = default_thread_consider_yield;
+
+#endif  // HAVE_THREADS && SIMPLE_THREADS
diff --git a/dtool/src/dtoolbase/dtoolbase_cc.h b/dtool/src/dtoolbase/dtoolbase_cc.h
index 303a15b360..8d23ee4d34 100644
--- a/dtool/src/dtoolbase/dtoolbase_cc.h
+++ b/dtool/src/dtoolbase/dtoolbase_cc.h
@@ -197,6 +197,28 @@ INLINE void operator delete[](void *ptr) {
 #endif  // REDEFINE_GLOBAL_OPERATOR_NEW
 #endif  // USE_MEMORY_NOWRAPPERS
 
+#if defined(HAVE_THREADS) && defined(SIMPLE_THREADS)
+// We need another forward-reference function to allow low-level code
+// to cooperatively yield the timeslice, in SIMPLE_THREADS mode.
+extern EXPCL_DTOOL void (*global_thread_yield)();
+extern EXPCL_DTOOL void (*global_thread_consider_yield)();
+
+INLINE void thread_yield() {
+  (*global_thread_yield)();
+}
+INLINE void thread_consider_yield() {
+  (*global_thread_consider_yield)();
+}
+
+#else
+
+INLINE void thread_yield() {
+}
+INLINE void thread_consider_yield() {
+}
+
+#endif  // HAVE_THREADS && SIMPLE_THREADS
+
 #if defined(USE_TAU) && defined(WIN32)
 // Hack around tau's lack of DLL export declarations for Profiler class.
 extern EXPCL_DTOOL bool __tau_shutdown;
diff --git a/dtool/src/dtoolutil/filename.cxx b/dtool/src/dtoolutil/filename.cxx
index 017a8e5eac..c2464d7d5a 100644
--- a/dtool/src/dtoolutil/filename.cxx
+++ b/dtool/src/dtoolutil/filename.cxx
@@ -1556,6 +1556,7 @@ scan_directory(vector_string &contents) const {
   }
 
   do {
+    thread_consider_yield();
     string filename = find_data.cFileName;
     if (filename != "." && filename != "..") {
       contents.push_back(filename);
@@ -1588,6 +1589,7 @@ scan_directory(vector_string &contents) const {
   struct dirent *d;
   d = readdir(root);
   while (d != (struct dirent *)NULL) {
+    thread_consider_yield();
     if (d->d_name[0] != '.') {
       contents.push_back(d->d_name);
     }
diff --git a/dtool/src/prc/encryptStreamBuf.cxx b/dtool/src/prc/encryptStreamBuf.cxx
index 1d1638e973..9ffac25ed1 100644
--- a/dtool/src/prc/encryptStreamBuf.cxx
+++ b/dtool/src/prc/encryptStreamBuf.cxx
@@ -186,6 +186,7 @@ open_read(istream *source, bool owns_source, const string &password) {
 
   _read_overflow_buffer = new unsigned char[_read_block_size];
   _in_read_overflow_buffer = 0;
+  thread_consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -300,6 +301,7 @@ open_write(ostream *dest, bool owns_dest, const string &password) {
   sw.append_data(iv, iv_length);
 
   _write_valid = true;
+  thread_consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -318,6 +320,7 @@ close_write() {
       unsigned char *write_buffer = (unsigned char *)alloca(_write_block_size);
       int bytes_written = 0;
       EVP_EncryptFinal(&_write_ctx, write_buffer, &bytes_written);
+      thread_consider_yield();
       
       _dest->write((const char *)write_buffer, bytes_written);
       
@@ -463,6 +466,7 @@ read_chars(char *start, size_t length) {
         _read_valid = false;
       }
     }
+    thread_consider_yield();
 
   } while (bytes_read == 0);
 
@@ -504,6 +508,7 @@ write_chars(const char *start, size_t length) {
       prc_cat.error() 
         << "Error encrypting stream.\n";
     }
+    thread_consider_yield();
     _dest->write((const char *)write_buffer, bytes_written);
   }
 }
diff --git a/panda/src/audiotraits/fmodAudioManager.cxx b/panda/src/audiotraits/fmodAudioManager.cxx
index b45947b608..f977352388 100644
--- a/panda/src/audiotraits/fmodAudioManager.cxx
+++ b/panda/src/audiotraits/fmodAudioManager.cxx
@@ -756,6 +756,7 @@ read_callback(void *handle, void *buffer, unsigned int size_bytes,
   istream *str = (istream *)handle;
   str->read((char *)buffer, size_bytes);
   (*bytes_read) = str->gcount();
+  thread_consider_yield();
 
   if (str->eof()) {
     if ((*bytes_read) == 0) {
diff --git a/panda/src/downloader/bioStreamBuf.cxx b/panda/src/downloader/bioStreamBuf.cxx
index 2c907fdb16..d88ed96064 100644
--- a/panda/src/downloader/bioStreamBuf.cxx
+++ b/panda/src/downloader/bioStreamBuf.cxx
@@ -170,6 +170,7 @@ underflow() {
     // BIO_read might return -1 or -2 on eof or error, so we have to
     // allow for negative numbers.
     int read_count = BIO_read(*_source, gptr(), buffer_size);
+    thread_consider_yield();
 
     if (read_count != (int)num_bytes) {
       // Oops, we didn't read what we thought we would.
@@ -239,6 +240,7 @@ write_chars(const char *start, size_t length) {
       downloader_cat.spam()
         << "wrote " << write_count << " bytes.\n";
     }
+    thread_consider_yield();
     while (write_count != (int)(length - wrote_so_far)) {
       if (write_count <= 0) {
         _is_closed = !BIO_should_retry(*_source);
@@ -275,6 +277,7 @@ write_chars(const char *start, size_t length) {
         downloader_cat.spam()
           << "continued, wrote " << write_count << " bytes.\n";
       }
+      thread_consider_yield();
     }
   }
 
diff --git a/panda/src/downloader/httpChannel.I b/panda/src/downloader/httpChannel.I
index ce26171a0c..ae87609ce9 100644
--- a/panda/src/downloader/httpChannel.I
+++ b/panda/src/downloader/httpChannel.I
@@ -666,7 +666,8 @@ send_extra_header(const string &key, const string &value) {
 INLINE bool HTTPChannel::
 get_document(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_get, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -683,7 +684,8 @@ get_document(const DocumentSpec &url) {
 INLINE bool HTTPChannel::
 get_subdocument(const DocumentSpec &url, size_t first_byte, size_t last_byte) {
   begin_request(HTTPEnum::M_get, url, string(), false, first_byte, last_byte);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -699,7 +701,8 @@ get_subdocument(const DocumentSpec &url, size_t first_byte, size_t last_byte) {
 INLINE bool HTTPChannel::
 get_header(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_head, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -712,7 +715,8 @@ get_header(const DocumentSpec &url) {
 INLINE bool HTTPChannel::
 post_form(const DocumentSpec &url, const string &body) {
   begin_request(HTTPEnum::M_post, url, body, false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -725,7 +729,8 @@ post_form(const DocumentSpec &url, const string &body) {
 INLINE bool HTTPChannel::
 put_document(const DocumentSpec &url, const string &body) {
   begin_request(HTTPEnum::M_put, url, body, false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -737,7 +742,8 @@ put_document(const DocumentSpec &url, const string &body) {
 INLINE bool HTTPChannel::
 delete_document(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_delete, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -751,7 +757,8 @@ delete_document(const DocumentSpec &url) {
 INLINE bool HTTPChannel::
 get_trace(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_trace, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
@@ -770,7 +777,8 @@ get_trace(const DocumentSpec &url) {
 INLINE bool HTTPChannel::
 connect_to(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_connect, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_connection_ready();
 }
 
@@ -784,7 +792,8 @@ connect_to(const DocumentSpec &url) {
 INLINE bool HTTPChannel::
 get_options(const DocumentSpec &url) {
   begin_request(HTTPEnum::M_options, url, string(), false, 0, 0);
-  run();
+  while (run()) {
+  }
   return is_valid();
 }
 
diff --git a/panda/src/downloader/httpChannel.cxx b/panda/src/downloader/httpChannel.cxx
index c111f2cee5..bb8c949434 100644
--- a/panda/src/downloader/httpChannel.cxx
+++ b/panda/src/downloader/httpChannel.cxx
@@ -58,8 +58,16 @@ HTTPChannel(HTTPClient *client) :
   _seconds_per_update = downloader_frequency;
   _max_updates_per_second = 1.0f / _seconds_per_update;
   _bytes_per_update = int(_max_bytes_per_second * _seconds_per_update);
+  
+  // _nonblocking is true if the socket is actually in non-blocking mode.
   _nonblocking = false;
 
+  // _wanted_nonblocking is true if the user specifically requested
+  // one of the non-blocking interfaces.  It is false if the socket is
+  // only incidentally non-blocking (for instance, because
+  // SIMPLE_THREADS is on).
+  _wanted_nonblocking = false;
+
   _want_ssl = false;
   _proxy_serves_document = false;
   _proxy_tunnel_now = false;
@@ -353,11 +361,12 @@ run() {
   }
 
   if (_started_download) {
-    if (_nonblocking && _download_throttle) {
+    if (_wanted_nonblocking && _download_throttle) {
       double now = TrueClock::get_global_ptr()->get_short_time();
       double elapsed = now - _last_run_time;
       if (elapsed < _seconds_per_update) {
         // Come back later.
+        thread_yield();
         return true;
       }
       int num_potential_updates = (int)(elapsed / _seconds_per_update);
@@ -370,16 +379,22 @@ run() {
           << _bytes_requested << "\n";
       }
     }
+
+    bool repeat_later;
     switch (_download_dest) {
     case DD_none:
-      return false;  // We're done.
+      repeat_later = false;  // We're done.
 
     case DD_file:
-      return run_download_to_file();
+      repeat_later = run_download_to_file();
 
     case DD_ram:
-      return run_download_to_ram();
+      repeat_later = run_download_to_ram();
     }
+    if (repeat_later) {
+      thread_yield();
+    }
+    return repeat_later;
   }
 
   if (downloader_cat.is_spam()) {
@@ -399,7 +414,7 @@ run() {
     // reestablish the connection if it has been dropped.
     if (_bio.is_null() && _state != S_try_next_proxy) {
       if (_connect_count > http_max_connect_count) {
-        // Too many connection attempts, just give up.  We should
+        // Too many connection attempts; just give up.  We should
         // never trigger this failsafe, since the code in each
         // individual case has similar logic to prevent more than two
         // consecutive lost connections.
@@ -534,6 +549,7 @@ run() {
       // We've reached our terminal state.
       return reached_done_state();
     }
+    thread_consider_yield();
   } while (!repeat_later || _bio.is_null());
 
   if (downloader_cat.is_spam()) {
@@ -541,6 +557,8 @@ run() {
       << "later run(), _state = " << _state
       << ", _done_state = " << _done_state << "\n";
   }
+
+  thread_yield();
   return true;
 }
 
@@ -631,7 +649,7 @@ download_to_file(const Filename &filename, bool subdocument_resumes) {
 
   _download_dest = DD_file;
 
-  if (_nonblocking) {
+  if (_wanted_nonblocking) {
     // In nonblocking mode, we can't start the download yet; that will
     // be done later as run() is called.
     return true;
@@ -643,7 +661,8 @@ download_to_file(const Filename &filename, bool subdocument_resumes) {
     return false;
   }
 
-  run();
+  while (run()) {
+  }
   return is_download_complete();
 }
 
@@ -687,7 +706,7 @@ download_to_ram(Ramfile *ramfile, bool subdocument_resumes) {
   _download_dest = DD_ram;
   _subdocument_resumes = (subdocument_resumes && _first_byte_delivered != 0);
 
-  if (_nonblocking) {
+  if (_wanted_nonblocking) {
     // In nonblocking mode, we can't start the download yet; that will
     // be done later as run() is called.
     return true;
@@ -699,7 +718,8 @@ download_to_ram(Ramfile *ramfile, bool subdocument_resumes) {
     return false;
   }
 
-  run();
+  while (run()) {
+  }
   return is_download_complete();
 }
 
@@ -2071,7 +2091,7 @@ bool HTTPChannel::
 run_download_to_file() {
   nassertr(_body_stream != (ISocketStream *)NULL, false);
 
-  bool do_throttle = _nonblocking && _download_throttle;
+  bool do_throttle = _wanted_nonblocking && _download_throttle;
 
   static const size_t buffer_size = 1024;
   char buffer[buffer_size];
@@ -2146,7 +2166,7 @@ run_download_to_ram() {
   nassertr(_body_stream != (ISocketStream *)NULL, false);
   nassertr(_download_to_ramfile != (Ramfile *)NULL, false);
 
-  bool do_throttle = _nonblocking && _download_throttle;
+  bool do_throttle = _wanted_nonblocking && _download_throttle;
 
   static const size_t buffer_size = 1024;
   char buffer[buffer_size];
@@ -2212,6 +2232,13 @@ begin_request(HTTPEnum::Method method, const DocumentSpec &url,
               size_t first_byte, size_t last_byte) {
   reset_for_new_request();
 
+  _wanted_nonblocking = nonblocking;
+#if defined(HAVE_THREADS) && defined(SIMPLE_THREADS)
+  // In the presence of SIMPLE_THREADS, we always use non-blocking
+  // I/O.  We simulate blocking by yielding the thread.
+  nonblocking = true;
+#endif
+
   // Get the set of proxies that are appropriate for this URL.
   _proxies.clear();
   _proxy_next_index = 0;
diff --git a/panda/src/downloader/httpChannel.h b/panda/src/downloader/httpChannel.h
index 396bd3fe91..32f2690afb 100644
--- a/panda/src/downloader/httpChannel.h
+++ b/panda/src/downloader/httpChannel.h
@@ -323,6 +323,7 @@ private:
   double _seconds_per_update;
   int _bytes_per_update;
   bool _nonblocking;
+  bool _wanted_nonblocking;
   string _send_extra_headers;
 
   DocumentSpec _document_spec;
diff --git a/panda/src/egg/eggMesher.cxx b/panda/src/egg/eggMesher.cxx
index 2360be8d8e..bdb007b395 100644
--- a/panda/src/egg/eggMesher.cxx
+++ b/panda/src/egg/eggMesher.cxx
@@ -26,6 +26,7 @@
 #include "config_egg.h"
 #include "eggGroupNode.h"
 #include "dcast.h"
+#include "thread.h"
 
 #include <stdlib.h>
 
@@ -325,6 +326,8 @@ do_mesh() {
 
   // Finally, do the longer strips.
   mesh_list(_strips);
+
+  Thread::consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
diff --git a/panda/src/egg/lexer.lxx b/panda/src/egg/lexer.lxx
index 515ddce0da..b902820769 100644
--- a/panda/src/egg/lexer.lxx
+++ b/panda/src/egg/lexer.lxx
@@ -14,6 +14,7 @@
 #include "indent.h"
 #include "pnotify.h"
 #include "pmutex.h"
+#include "thread.h"
 
 #include <math.h>
 
@@ -188,6 +189,7 @@ input_chars(char *buffer, int &result, int max_size) {
     // End of file or I/O error.
     result = 0;
   }
+  Thread::consider_yield();
 }
 #undef YY_INPUT
 #define YY_INPUT(buffer, result, max_size) input_chars(buffer, result, max_size)
diff --git a/panda/src/egg2pg/eggLoader.cxx b/panda/src/egg2pg/eggLoader.cxx
index 648e797e09..8f6bec27b7 100644
--- a/panda/src/egg2pg/eggLoader.cxx
+++ b/panda/src/egg2pg/eggLoader.cxx
@@ -92,6 +92,7 @@
 #include "transformBlend.h"
 #include "sparseArray.h"
 #include "bitArray.h"
+#include "thread.h"
 
 #include <ctype.h>
 #include <algorithm>
@@ -2321,6 +2322,7 @@ make_vertex_data(const EggRenderState *render_state,
     (VertexPoolData::value_type(vpt, vertex_data)).second;
   nassertr(inserted, vertex_data);
 
+  Thread::consider_yield();
   return vertex_data;
 }
 
diff --git a/panda/src/express/virtualFile.cxx b/panda/src/express/virtualFile.cxx
index 10c8e22870..b65bf7d41c 100644
--- a/panda/src/express/virtualFile.cxx
+++ b/panda/src/express/virtualFile.cxx
@@ -284,6 +284,7 @@ read_file(istream *in, string &result) {
   in->read(buffer, buffer_size);
   size_t count = in->gcount();
   while (count != 0) {
+    thread_consider_yield();
     result_vec.insert(result_vec.end(), buffer, buffer + count);
     in->read(buffer, buffer_size);
     count = in->gcount();
@@ -309,6 +310,7 @@ read_file(istream *in, string &result, size_t max_bytes) {
   in->read(buffer, min(buffer_size, max_bytes));
   size_t count = in->gcount();
   while (count != 0) {
+    thread_consider_yield();
     nassertr(count <= max_bytes, false);
     result_vec.insert(result_vec.end(), buffer, buffer + count);
     max_bytes -= count;
diff --git a/panda/src/express/zStreamBuf.cxx b/panda/src/express/zStreamBuf.cxx
index 778cd770ba..89bffab03b 100644
--- a/panda/src/express/zStreamBuf.cxx
+++ b/panda/src/express/zStreamBuf.cxx
@@ -86,6 +86,7 @@ open_read(istream *source, bool owns_source) {
     show_zlib_error("inflateInit", result, _z_source);
     close_read();
   }
+  thread_consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -101,6 +102,7 @@ close_read() {
     if (result < 0) {
       show_zlib_error("inflateEnd", result, _z_source);
     }
+    thread_consider_yield();
 
     if (_owns_source) {
       delete _source;
@@ -130,6 +132,7 @@ open_write(ostream *dest, bool owns_dest, int compression_level) {
     show_zlib_error("deflateInit", result, _z_dest);
     close_write();
   }
+  thread_consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -148,6 +151,7 @@ close_write() {
     if (result < 0) {
       show_zlib_error("deflateEnd", result, _z_dest);
     }
+    thread_consider_yield();
 
     if (_owns_dest) {
       delete _dest;
@@ -260,6 +264,7 @@ read_chars(char *start, size_t length) {
       _z_source.avail_in = read_count;
     }
     int result = inflate(&_z_source, flush);
+    thread_consider_yield();
     size_t bytes_read = length - _z_source.avail_out;
 
     if (result == Z_STREAM_END) {
@@ -302,6 +307,7 @@ write_chars(const char *start, size_t length, int flush) {
   if (result < 0 && result != Z_BUF_ERROR) {
     show_zlib_error("deflate", result, _z_dest);
   }
+  thread_consider_yield();
 
   while (_z_dest.avail_in != 0) {
     if (_z_dest.avail_out != compress_buffer_size) {
@@ -313,6 +319,7 @@ write_chars(const char *start, size_t length, int flush) {
     if (result < 0) {
       show_zlib_error("deflate", result, _z_dest);
     }
+    thread_consider_yield();
   }
 
   while (_z_dest.avail_out != compress_buffer_size) {
@@ -323,6 +330,7 @@ write_chars(const char *start, size_t length, int flush) {
     if (result < 0 && result != Z_BUF_ERROR) {
       show_zlib_error("deflate", result, _z_dest);
     }
+    thread_consider_yield();
   }
 }
 
diff --git a/panda/src/gobj/vertexDataSaveFile.cxx b/panda/src/gobj/vertexDataSaveFile.cxx
index 148a58f110..f3d3b6428d 100644
--- a/panda/src/gobj/vertexDataSaveFile.cxx
+++ b/panda/src/gobj/vertexDataSaveFile.cxx
@@ -93,7 +93,13 @@ VertexDataSaveFile(const Filename &directory, const string &prefix,
 
 #else
     // Posix case.
-    _fd = open(os_specific.c_str(), O_RDWR | O_CREAT, 0666);
+    int flags = O_RDWR | O_CREAT;
+#if defined(HAVE_THREADS) && defined(SIMPLE_THREADS)
+    // In SIMPLE_THREADS mode, we use non-blocking I/O.
+    flags |= O_NONBLOCK;
+#endif
+
+    _fd = open(os_specific.c_str(), flags, 0666);
     if (_fd == -1) {
       // Couldn't open the file: permissions problem or bad directory.
       if (!_filename.exists()) {
@@ -210,11 +216,21 @@ write_data(const unsigned char *data, size_t size, bool compressed) {
       return false;
     }
 
-    ssize_t result = ::write(_fd, data, size);
-    if (result != (ssize_t)size) {
-      gobj_cat.error()
-        << "Error writing " << size << " bytes to save file.  Disk full?\n";
-      return NULL;
+    while (size > 0) {
+      ssize_t result = ::write(_fd, data, size);
+      if (result == -1) {
+        if (errno == EAGAIN) {
+          Thread::force_yield();
+        } else {
+          gobj_cat.error()
+            << "Error writing " << size << " bytes to save file.  Disk full?\n";
+          return NULL;
+        }
+      }
+
+      Thread::consider_yield();
+      data += result;
+      size -= result;
     }
 #endif  // _WIN32
 
@@ -263,11 +279,21 @@ read_data(unsigned char *data, size_t size, VertexDataSaveBlock *block) {
       << "Error seeking to position " << block->get_start() << " in save file.\n";
     return false;
   }
-  ssize_t result = read(_fd, data, size);
-  if (result != (ssize_t)size) {
-    gobj_cat.error()
-      << "Error reading " << size << " bytes from save file.\n";
-    return false;
+  while (size > 0) {
+    ssize_t result = read(_fd, data, size);
+    if (result == -1) {
+      if (errno == EAGAIN) {
+        Thread::force_yield();
+      } else {
+        gobj_cat.error()
+          << "Error reading " << size << " bytes from save file.\n";
+        return false;
+      }
+    }
+
+    Thread::consider_yield();
+    data += result;
+    size -= result;
   }
 #endif  // _WIN32
 
diff --git a/panda/src/net/connectionReader.cxx b/panda/src/net/connectionReader.cxx
index 74490b08ba..57cb2b0bac 100644
--- a/panda/src/net/connectionReader.cxx
+++ b/panda/src/net/connectionReader.cxx
@@ -102,7 +102,7 @@ ConnectionReader::
 ConnectionReader(ConnectionManager *manager, int num_threads) :
   _manager(manager)
 {
-#ifndef HAVE_THREADS
+#if !defined(HAVE_THREADS) || defined(SIMPLE_THREADS)
 #ifndef NDEBUG
   if (num_threads != 0) {
     net_cat.error()
diff --git a/panda/src/net/connectionWriter.cxx b/panda/src/net/connectionWriter.cxx
index feb24a3342..c142b4c309 100644
--- a/panda/src/net/connectionWriter.cxx
+++ b/panda/src/net/connectionWriter.cxx
@@ -61,11 +61,13 @@ ConnectionWriter::
 ConnectionWriter(ConnectionManager *manager, int num_threads) :
   _manager(manager)
 {
-#ifndef HAVE_THREADS
+#if !defined(HAVE_THREADS) || defined(SIMPLE_THREADS)
   // Although this code is written to use thread-locking primitives
   // regardless of the definition of HAVE_THREADS, it is not safe to
   // spawn multiple threads when HAVE_THREADS is not true, since we
-  // might be using a non-thread-safe malloc scheme.
+  // might be using a non-thread-safe malloc scheme.  Also, there is
+  // no point in using threads for this kind of I/O if SIMPLE_THREADS
+  // is defined.
 #ifndef NDEBUG
   if (num_threads != 0) {
     net_cat.error()
diff --git a/panda/src/pgraph/sceneGraphReducer.cxx b/panda/src/pgraph/sceneGraphReducer.cxx
index 3205b1576b..cee6a428d6 100644
--- a/panda/src/pgraph/sceneGraphReducer.cxx
+++ b/panda/src/pgraph/sceneGraphReducer.cxx
@@ -25,6 +25,7 @@
 #include "plist.h"
 #include "pmap.h"
 #include "geomNode.h"
+#include "thread.h"
 
 PStatCollector SceneGraphReducer::_flatten_collector("*:Flatten:flatten");
 PStatCollector SceneGraphReducer::_apply_collector("*:Flatten:apply");
@@ -771,7 +772,8 @@ r_collect_vertex_data(PandaNode *node, int collect_bits,
         r_collect_vertex_data(children.get_child(i), collect_bits, transformer);
     }
   }
-    
+
+  Thread::consider_yield();
   return num_created;
 }
 
@@ -840,6 +842,7 @@ r_unify(PandaNode *node, int max_indices) {
   for (int i = 0; i < num_children; ++i) {
     r_unify(children.get_child(i), max_indices);
   }
+  Thread::consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
diff --git a/panda/src/pipeline/threadSimpleManager.cxx b/panda/src/pipeline/threadSimpleManager.cxx
index 9027a695e0..8b7bec8954 100644
--- a/panda/src/pipeline/threadSimpleManager.cxx
+++ b/panda/src/pipeline/threadSimpleManager.cxx
@@ -41,6 +41,11 @@ ThreadSimpleManager() {
   _current_thread = NULL;
   _clock = TrueClock::get_global_ptr();
   _waiting_for_exit = NULL;
+
+  // Install these global pointers so very low-level code (code
+  // defined before the pipeline directory) can yield when necessary.
+  global_thread_yield = &Thread::force_yield;
+  global_thread_consider_yield = &Thread::consider_yield;
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -53,7 +58,9 @@ ThreadSimpleManager() {
 ////////////////////////////////////////////////////////////////////
 void ThreadSimpleManager::
 enqueue_ready(ThreadSimpleImpl *thread) {
-  _ready.push_back(thread);
+  // We actually add it to _next_ready, so that we can tell when we
+  // have processed every thread in a given epoch.
+  _next_ready.push_back(thread);
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -353,9 +360,26 @@ choose_next_context() {
 
   // Choose a new thread to execute.
   while (_ready.empty()) {
-    // No threads are ready.  They must all be sleeping.
-    if (_sleeping.empty()) {
-      // No threads at all!
+    if (!_next_ready.empty()) {
+      // We've finished an epoch.
+      _ready.swap(_next_ready);
+      system_yield();
+
+    } else if (!_sleeping.empty()) {
+      // All threads are sleeping.
+      double wait = _sleeping.front()->_start_time - now;
+      if (wait > 0.0) {
+        if (thread_cat.is_debug()) {
+          thread_cat.debug()
+            << "Sleeping all threads " << wait << " seconds\n";
+        }
+        system_sleep(wait);
+      }
+      now = get_current_time();
+      wake_sleepers(now);
+
+    } else {
+      // No threads are ready!
       if (!_blocked.empty()) {
         thread_cat.error()
           << "Deadlock!  All threads blocked.\n";
@@ -372,24 +396,13 @@ choose_next_context() {
         break;
       }
 
-      // No threads are queued anywhere.  This is kind of an error,
-      // since normally the main thread, at least, should be queued
-      // somewhere.
+      // No threads are queued anywhere.  This is some kind of
+      // internal error, since normally the main thread, at least,
+      // should be queued somewhere.
       thread_cat.error()
         << "All threads disappeared!\n";
       exit(0);
     }
-
-    double wait = _sleeping.front()->_start_time - now;
-    if (wait > 0.0) {
-      if (thread_cat.is_debug()) {
-        thread_cat.debug()
-          << "Sleeping all threads " << wait << " seconds\n";
-      }
-      system_sleep(wait);
-    }
-    now = get_current_time();
-    wake_sleepers(now);
   }
 
   _current_thread = _ready.front();
@@ -454,6 +467,25 @@ system_sleep(double seconds) {
 #endif  // WIN32
 }
 
+////////////////////////////////////////////////////////////////////
+//     Function: ThreadSimpleManager::system_yield
+//       Access: Private, Static
+//  Description: Calls the appropriate system function to yield
+//               the whole process to any other system processes.
+////////////////////////////////////////////////////////////////////
+void ThreadSimpleManager::
+system_yield() {
+#ifdef WIN32
+  Sleep(0);
+
+#else
+  struct timespec rqtp;
+  rqtp.tv_sec = 0;
+  rqtp.tv_nsec = 0;
+  nanosleep(&rqtp, NULL);
+#endif  // WIN32
+}
+
 ////////////////////////////////////////////////////////////////////
 //     Function: ThreadSimpleManager::report_deadlock
 //       Access: Private
diff --git a/panda/src/pipeline/threadSimpleManager.h b/panda/src/pipeline/threadSimpleManager.h
index 69c15159ff..6814e95289 100644
--- a/panda/src/pipeline/threadSimpleManager.h
+++ b/panda/src/pipeline/threadSimpleManager.h
@@ -79,6 +79,7 @@ private:
   void choose_next_context();
   void wake_sleepers(double now);
   static void system_sleep(double seconds);
+  static void system_yield();
   void report_deadlock();
 
   // STL function object to sort the priority queue of sleeping threads.
@@ -98,6 +99,7 @@ private:
 
   // FIFO list of ready threads.
   FifoThreads _ready;
+  FifoThreads _next_ready;
 
   typedef pmap<BlockerSimple *, FifoThreads> Blocked;
   Blocked _blocked;
diff --git a/panda/src/pnmimage/pnmReader.cxx b/panda/src/pnmimage/pnmReader.cxx
index e24671a2be..d559466f23 100644
--- a/panda/src/pnmimage/pnmReader.cxx
+++ b/panda/src/pnmimage/pnmReader.cxx
@@ -18,6 +18,7 @@
 
 #include "pnmReader.h"
 #include "virtualFileSystem.h"
+#include "thread.h"
 
 ////////////////////////////////////////////////////////////////////
 //     Function: PNMReader::Destructor
@@ -97,6 +98,7 @@ read_data(xel *array, xelval *alpha) {
     int y;
     for (y = 0; y < _y_size; ++y) {
       if (!read_row(array + y * _x_size, alpha + y * _x_size, _x_size, _y_size)) {
+        Thread::consider_yield();
         return y;
       }
     }
@@ -129,6 +131,7 @@ read_data(xel *array, xelval *alpha) {
       for (int yi = 0; yi < y_reduction; ++yi) {
         // OK, read a row.  This reads the original, full-size row.
         if (!read_row(orig_row_array, orig_row_alpha, _orig_x_size, _orig_y_size)) {
+          Thread::consider_yield();
           return y;
         }
 
diff --git a/panda/src/pnmimage/pnmWriter.cxx b/panda/src/pnmimage/pnmWriter.cxx
index dedbd91fd6..4a9a32e15f 100644
--- a/panda/src/pnmimage/pnmWriter.cxx
+++ b/panda/src/pnmimage/pnmWriter.cxx
@@ -17,6 +17,7 @@
 ////////////////////////////////////////////////////////////////////
 
 #include "pnmWriter.h"
+#include "thread.h"
 
 ////////////////////////////////////////////////////////////////////
 //     Function: PNMWriter::Destructor
@@ -66,6 +67,7 @@ write_data(xel *array, xelval *alpha) {
   int y;
   for (y = 0; y < _y_size; y++) {
     if (!write_row(array + y * _x_size, alpha + y * _x_size)) {
+      Thread::consider_yield();
       return y;
     }
   }
diff --git a/panda/src/pnmimagetypes/pnmFileTypeBMPWriter.cxx b/panda/src/pnmimagetypes/pnmFileTypeBMPWriter.cxx
index 25297ce8fb..bb84499d06 100644
--- a/panda/src/pnmimagetypes/pnmFileTypeBMPWriter.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypeBMPWriter.cxx
@@ -25,6 +25,7 @@
 #include "bmp.h"
 #include "ppmcmap.h"
 #include "pnmbitio.h"
+#include "thread.h"
 
 // Much code in this file is borrowed from Netpbm, specifically ppmtobmp.c.
 /*
@@ -629,6 +630,7 @@ write_data(xel *array, xelval *) {
     BMPEncode(_file, classv, _x_size, _y_size, pixels, colors, cht,
               Red, Green, Blue);
   }
+  Thread::consider_yield();
 
   return _y_size;
 }
diff --git a/panda/src/pnmimagetypes/pnmFileTypeJPGReader.cxx b/panda/src/pnmimagetypes/pnmFileTypeJPGReader.cxx
index 5618cbf815..3edee02130 100644
--- a/panda/src/pnmimagetypes/pnmFileTypeJPGReader.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypeJPGReader.cxx
@@ -21,7 +21,7 @@
 #ifdef HAVE_JPEG
 
 #include "config_pnmimagetypes.h"
-
+#include "thread.h"
 
 //
 // The following bit of code, for setting up jpeg_istream_src(), was
@@ -126,6 +126,7 @@ fill_input_buffer (j_decompress_ptr cinfo)
 
   src->infile->read((char *)src->buffer, INPUT_BUF_SIZE);
   nbytes = src->infile->gcount();
+  Thread::consider_yield();
 
   if (nbytes <= 0) {
     if (src->start_of_file)     /* Treat empty input file as fatal error */
diff --git a/panda/src/pnmimagetypes/pnmFileTypeJPGWriter.cxx b/panda/src/pnmimagetypes/pnmFileTypeJPGWriter.cxx
index e2a9b76922..3b1ed9911f 100644
--- a/panda/src/pnmimagetypes/pnmFileTypeJPGWriter.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypeJPGWriter.cxx
@@ -24,6 +24,7 @@
 
 #include "pnmImage.h"
 #include "pnmWriter.h"
+#include "thread.h"
 
 
 //
@@ -122,6 +123,7 @@ empty_output_buffer (j_compress_ptr cinfo)
 
   dest->pub.next_output_byte = dest->buffer;
   dest->pub.free_in_buffer = OUTPUT_BUF_SIZE;
+  Thread::consider_yield();
 
   return TRUE;
 }
@@ -148,6 +150,7 @@ term_destination (j_compress_ptr cinfo)
       ERREXIT(cinfo, JERR_FILE_WRITE);
   }
   dest->outfile->flush();
+  Thread::consider_yield();
   /* Make sure we wrote the output file OK */
   if (dest->outfile->fail())
     ERREXIT(cinfo, JERR_FILE_WRITE);
diff --git a/panda/src/pnmimagetypes/pnmFileTypePNG.cxx b/panda/src/pnmimagetypes/pnmFileTypePNG.cxx
index 1ba6861937..fb20afd580 100755
--- a/panda/src/pnmimagetypes/pnmFileTypePNG.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypePNG.cxx
@@ -24,6 +24,7 @@
 
 #include "pnmFileTypeRegistry.h"
 #include "bamReader.h"
+#include "thread.h"
 
 static const char * const extensions_png[] = {
   "png"
@@ -453,6 +454,7 @@ png_read_data(png_structp png_ptr, png_bytep data, png_size_t length) {
       << "Didn't read enough bytes.\n";
     // Is there no way to indicate a read failure to libpng?
   }
+  Thread::consider_yield();
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -774,6 +776,7 @@ write_data(xel *array, xelval *alpha_data) {
 
     nassertr(dest <= row + row_byte_length, yi);
     png_write_row(_png, row);
+    Thread::consider_yield();
   }
 
   delete[] row;
@@ -831,7 +834,7 @@ make_png_bit_depth(int bit_depth) {
 ////////////////////////////////////////////////////////////////////
 //     Function: PNMFileTypePNG::Writer::png_write_data
 //       Access: Private, Static
-//  Description: A callback handler that PNG uses to write data from
+//  Description: A callback handler that PNG uses to write data to
 //               the iostream.
 ////////////////////////////////////////////////////////////////////
 void PNMFileTypePNG::Writer::
@@ -848,7 +851,7 @@ png_write_data(png_structp png_ptr, png_bytep data, png_size_t length) {
 ////////////////////////////////////////////////////////////////////
 //     Function: PNMFileTypePNG::Writer::png_flush_data
 //       Access: Private, Static
-//  Description: A callback handler that PNG uses to write data from
+//  Description: A callback handler that PNG uses to write data to
 //               the iostream.
 ////////////////////////////////////////////////////////////////////
 void PNMFileTypePNG::Writer::
diff --git a/panda/src/pnmimagetypes/pnmFileTypeSGIReader.cxx b/panda/src/pnmimagetypes/pnmFileTypeSGIReader.cxx
index f364a96a90..40948e3272 100644
--- a/panda/src/pnmimagetypes/pnmFileTypeSGIReader.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypeSGIReader.cxx
@@ -484,6 +484,7 @@ read_bytes(istream *ifp,
       readerr(ifp);
       memset(buf+r, 0, n-r);
     }
+    Thread::consider_yield();
 }
 
 
diff --git a/panda/src/pnmimagetypes/pnmFileTypeTIFF.cxx b/panda/src/pnmimagetypes/pnmFileTypeTIFF.cxx
index 77f907f61b..2221e2ce68 100644
--- a/panda/src/pnmimagetypes/pnmFileTypeTIFF.cxx
+++ b/panda/src/pnmimagetypes/pnmFileTypeTIFF.cxx
@@ -93,6 +93,7 @@ static tsize_t
 istream_read(thandle_t fd, tdata_t buf, tsize_t size) {
   istream *in = (istream *)fd;
   in->read((char *)buf, size);
+  Thread::consider_yield();
   return in->gcount();
 }
 
@@ -100,6 +101,7 @@ static tsize_t
 ostream_write(thandle_t fd, tdata_t buf, tsize_t size) {
   ostream *out = (ostream *)fd;
   out->write((char *)buf, size);
+  Thread::consider_yield();
   return out->fail() ? (tsize_t)0 : size;
 }
 
diff --git a/panda/src/putil/datagramInputFile.cxx b/panda/src/putil/datagramInputFile.cxx
index 5f6110919a..38e3679274 100644
--- a/panda/src/putil/datagramInputFile.cxx
+++ b/panda/src/putil/datagramInputFile.cxx
@@ -24,6 +24,7 @@
 #include "config_express.h"
 #include "virtualFileSystem.h"
 #include "streamReader.h"
+#include "thread.h"
 
 ////////////////////////////////////////////////////////////////////
 //     Function: DatagramInputFile::open
@@ -112,6 +113,7 @@ read_header(string &header, size_t num_bytes) {
   }
 
   header = string(buffer, num_bytes);
+  Thread::consider_yield();
   return true;
 }
 
@@ -172,6 +174,7 @@ get_datagram(Datagram &data) {
 
     data = Datagram(buffer, num_bytes);
   }
+  Thread::consider_yield();
 
   return true;
 }
diff --git a/panda/src/putil/datagramOutputFile.cxx b/panda/src/putil/datagramOutputFile.cxx
index fe7d6b38e4..cb2de8a65b 100644
--- a/panda/src/putil/datagramOutputFile.cxx
+++ b/panda/src/putil/datagramOutputFile.cxx
@@ -101,6 +101,7 @@ write_header(const string &header) {
   nassertr(!_wrote_first_datagram, false);
 
   _out->write(header.data(), header.size());
+  thread_consider_yield();
   return !_out->fail();
 }
 
@@ -121,6 +122,7 @@ put_datagram(const Datagram &data) {
 
   // Now, write the datagram itself.
   _out->write((const char *)data.get_data(), data.get_length());
+  thread_consider_yield();
 
   return !_out->fail();
 }