mirror of
https://github.com/cuberite/libevent.git
synced 2025-09-09 12:28:19 -04:00
Replace big chain of if/thens in epoll.c with a table lookup
This should save a bunch of branches by doing instead a lookup in a nice static table. To ensure correctness, the table is generated from a Python script, included with this commit.
This commit is contained in:
Nick Mathewson
parent
b8b8aa560c
commit
8c83eb6948
249
epoll.c
249
epoll.c
@ -167,6 +167,170 @@ epoll_op_to_string(int op)
|
||||
"???";
|
||||
}
|
||||
|
||||
/*
|
||||
Here are the values we're masking off to decide what operations to do.
|
||||
Note that since EV_READ|EV_WRITE.
|
||||
|
||||
Note also that this table is a little sparse, since ADD+DEL is
|
||||
nonsensical ("xxx" in the list below.)
|
||||
|
||||
Note also also that we are shifting old_events by only 3 bits, since
|
||||
EV_READ is 2 and EV_WRITE is 4.
|
||||
|
||||
The table was auto-generated with a python script, according to this
|
||||
pseudocode:
|
||||
|
||||
If either the read or the write change is add+del:
|
||||
This is impossible; Set op==-1, events=0.
|
||||
Else, if either the read or the write change is add:
|
||||
Set events to 0.
|
||||
If the read change is add, or
|
||||
(the read change is not del, and ev_read is in old_events):
|
||||
Add EPOLLIN to events.
|
||||
If the write change is add, or
|
||||
(the write change is not del, and ev_write is in old_events):
|
||||
Add EPOLLOUT to events.
|
||||
|
||||
If old_events is set:
|
||||
Set op to EPOLL_CTL_MOD [*1,*2]
|
||||
Else:
|
||||
Set op to EPOLL_CTL_ADD [*3]
|
||||
|
||||
Else, if the read or the write change is del:
|
||||
Set op to EPOLL_CTL_DEL.
|
||||
If the read change is del:
|
||||
If the write change is del:
|
||||
Set events to EPOLLIN|EPOLLOUT
|
||||
Else if ev_write is in old_events:
|
||||
Set events to EPOLLOUT
|
||||
Set op to EPOLL_CTL_MOD
|
||||
Else
|
||||
Set events to EPOLLIN
|
||||
Else:
|
||||
{The write change is del.}
|
||||
If ev_read is in old_events:
|
||||
Set events to EPOLLIN
|
||||
Set op to EPOLL_CTL_MOD
|
||||
Else:
|
||||
Set the events to EPOLLOUT
|
||||
|
||||
Else:
|
||||
There is no read or write change; set op to 0 and events to 0.
|
||||
|
||||
The logic is a little tricky, since we had no events set on the fd before,
|
||||
we need to set op="ADD" and set events=the events we want to add. If we
|
||||
had any events set on the fd before, and we want any events to remain on
|
||||
the fd, we need to say op="MOD" and set events=the events we want to
|
||||
remain. But if we want to delete the last event, we say op="DEL" and
|
||||
set events=(any non-null pointer).
|
||||
[*1] This MOD is only a guess. MOD might fail with ENOENT if the file was
|
||||
closed and a new file was opened with the same fd. If so, we'll retry
|
||||
with ADD.
|
||||
|
||||
[*2] We can't replace this with a no-op even if old_events is the same as
|
||||
the new events: if the file was closed and reopened, we need to retry
|
||||
with an ADD. (We do a MOD in this case since "no change" is more
|
||||
common than "close and reopen", so we'll usually wind up doing 1
|
||||
syscalls instead of 2.)
|
||||
|
||||
[*3] This ADD is only a guess. There is a fun Linux kernel issue where if
|
||||
you have two fds for the same file (via dup) and you ADD one to an
|
||||
epfd, then close it, then re-create it with the same fd (via dup2 or an
|
||||
unlucky dup), then try to ADD it again, you'll get an EEXIST, since the
|
||||
struct epitem is not actually removed from the struct eventpoll until
|
||||
the file itself is closed.
|
||||
|
||||
EV_CHANGE_ADD==1
|
||||
EV_CHANGE_DEL==2
|
||||
EV_READ ==2
|
||||
EV_WRITE ==4
|
||||
Bit 0: read change is add
|
||||
Bit 1: read change is del
|
||||
Bit 2: write change is add
|
||||
Bit 3: write change is del
|
||||
Bit 4: old events had EV_READ
|
||||
Bit 5: old events had EV_WRITE
|
||||
*/
|
||||
|
||||
#define INDEX(c) \
|
||||
( (((c)->read_change&(EV_CHANGE_ADD|EV_CHANGE_DEL))) | \
|
||||
(((c)->write_change&(EV_CHANGE_ADD|EV_CHANGE_DEL)) << 2) | \
|
||||
(((c)->old_events&(EV_READ|EV_WRITE)) << 3) )
|
||||
|
||||
#if EV_READ != 2 || EV_WRITE != 4 || EV_CHANGE_ADD != 1 || EV_CHANGE_DEL != 2
|
||||
#error "Libevent's internals changed! Regenerate the op_table in epoll.c"
|
||||
#endif
|
||||
|
||||
static const struct operation {
|
||||
int events;
|
||||
int op;
|
||||
} op_table[] = {
|
||||
{ 0, 0 }, /* old= 0, write: 0, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_ADD }, /* old= 0, write: 0, read:add */
|
||||
{ EPOLLIN, EPOLL_CTL_DEL }, /* old= 0, write: 0, read:del */
|
||||
{ 0, -1 }, /* old= 0, write: 0, read:xxx */
|
||||
{ EPOLLOUT, EPOLL_CTL_ADD }, /* old= 0, write:add, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_ADD },/* old= 0, write:add, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_ADD }, /* old= 0, write:add, read:del */
|
||||
{ 0, -1 }, /* old= 0, write:add, read:xxx */
|
||||
{ EPOLLOUT, EPOLL_CTL_DEL }, /* old= 0, write:del, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_ADD }, /* old= 0, write:del, read:add */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_DEL },/* old= 0, write:del, read:del */
|
||||
{ 0, -1 }, /* old= 0, write:del, read:xxx */
|
||||
{ 0, -1 }, /* old= 0, write:xxx, read: 0 */
|
||||
{ 0, -1 }, /* old= 0, write:xxx, read:add */
|
||||
{ 0, -1 }, /* old= 0, write:xxx, read:del */
|
||||
{ 0, -1 }, /* old= 0, write:xxx, read:xxx */
|
||||
{ 0, 0 }, /* old= r, write: 0, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old= r, write: 0, read:add */
|
||||
{ EPOLLIN, EPOLL_CTL_DEL }, /* old= r, write: 0, read:del */
|
||||
{ 0, -1 }, /* old= r, write: 0, read:xxx */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old= r, write:add, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old= r, write:add, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old= r, write:add, read:del */
|
||||
{ 0, -1 }, /* old= r, write:add, read:xxx */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old= r, write:del, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old= r, write:del, read:add */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_DEL },/* old= r, write:del, read:del */
|
||||
{ 0, -1 }, /* old= r, write:del, read:xxx */
|
||||
{ 0, -1 }, /* old= r, write:xxx, read: 0 */
|
||||
{ 0, -1 }, /* old= r, write:xxx, read:add */
|
||||
{ 0, -1 }, /* old= r, write:xxx, read:del */
|
||||
{ 0, -1 }, /* old= r, write:xxx, read:xxx */
|
||||
{ 0, 0 }, /* old= w, write: 0, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old= w, write: 0, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old= w, write: 0, read:del */
|
||||
{ 0, -1 }, /* old= w, write: 0, read:xxx */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old= w, write:add, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old= w, write:add, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old= w, write:add, read:del */
|
||||
{ 0, -1 }, /* old= w, write:add, read:xxx */
|
||||
{ EPOLLOUT, EPOLL_CTL_DEL }, /* old= w, write:del, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old= w, write:del, read:add */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_DEL },/* old= w, write:del, read:del */
|
||||
{ 0, -1 }, /* old= w, write:del, read:xxx */
|
||||
{ 0, -1 }, /* old= w, write:xxx, read: 0 */
|
||||
{ 0, -1 }, /* old= w, write:xxx, read:add */
|
||||
{ 0, -1 }, /* old= w, write:xxx, read:del */
|
||||
{ 0, -1 }, /* old= w, write:xxx, read:xxx */
|
||||
{ 0, 0 }, /* old=rw, write: 0, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old=rw, write: 0, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old=rw, write: 0, read:del */
|
||||
{ 0, -1 }, /* old=rw, write: 0, read:xxx */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old=rw, write:add, read: 0 */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_MOD },/* old=rw, write:add, read:add */
|
||||
{ EPOLLOUT, EPOLL_CTL_MOD }, /* old=rw, write:add, read:del */
|
||||
{ 0, -1 }, /* old=rw, write:add, read:xxx */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old=rw, write:del, read: 0 */
|
||||
{ EPOLLIN, EPOLL_CTL_MOD }, /* old=rw, write:del, read:add */
|
||||
{ EPOLLIN|EPOLLOUT, EPOLL_CTL_DEL },/* old=rw, write:del, read:del */
|
||||
{ 0, -1 }, /* old=rw, write:del, read:xxx */
|
||||
{ 0, -1 }, /* old=rw, write:xxx, read: 0 */
|
||||
{ 0, -1 }, /* old=rw, write:xxx, read:add */
|
||||
{ 0, -1 }, /* old=rw, write:xxx, read:del */
|
||||
{ 0, -1 }, /* old=rw, write:xxx, read:xxx */
|
||||
};
|
||||
|
||||
static int
|
||||
epoll_apply_one_change(struct event_base *base,
|
||||
struct epollop *epollop,
|
||||
@ -174,87 +338,20 @@ epoll_apply_one_change(struct event_base *base,
|
||||
{
|
||||
struct epoll_event epev;
|
||||
int op, events = 0;
|
||||
int idx;
|
||||
|
||||
if (1) {
|
||||
/* The logic here is a little tricky. If we had no events set
|
||||
on the fd before, we need to set op="ADD" and set
|
||||
events=the events we want to add. If we had any events set
|
||||
on the fd before, and we want any events to remain on the
|
||||
fd, we need to say op="MOD" and set events=the events we
|
||||
want to remain. But if we want to delete the last event,
|
||||
we say op="DEL" and set events=the remaining events. What
|
||||
fun!
|
||||
*/
|
||||
idx = INDEX(ch);
|
||||
op = op_table[idx].op;
|
||||
events = op_table[idx].events;
|
||||
|
||||
/* TODO: Turn this into a switch or a table lookup. */
|
||||
|
||||
if ((ch->read_change & EV_CHANGE_ADD) ||
|
||||
(ch->write_change & EV_CHANGE_ADD)) {
|
||||
/* If we are adding anything at all, we'll want to do
|
||||
* either an ADD or a MOD. */
|
||||
events = 0;
|
||||
op = EPOLL_CTL_ADD;
|
||||
if (ch->read_change & EV_CHANGE_ADD) {
|
||||
events |= EPOLLIN;
|
||||
} else if (ch->read_change & EV_CHANGE_DEL) {
|
||||
;
|
||||
} else if (ch->old_events & EV_READ) {
|
||||
events |= EPOLLIN;
|
||||
}
|
||||
if (ch->write_change & EV_CHANGE_ADD) {
|
||||
events |= EPOLLOUT;
|
||||
} else if (ch->write_change & EV_CHANGE_DEL) {
|
||||
;
|
||||
} else if (ch->old_events & EV_WRITE) {
|
||||
events |= EPOLLOUT;
|
||||
}
|
||||
if ((ch->read_change|ch->write_change) & EV_ET)
|
||||
events |= EPOLLET;
|
||||
|
||||
if (ch->old_events) {
|
||||
/* If MOD fails, we retry as an ADD, and if
|
||||
* ADD fails we will retry as a MOD. So the
|
||||
* only hard part here is to guess which one
|
||||
* will work. As a heuristic, we'll try
|
||||
* MOD first if we think there were old
|
||||
* events and ADD if we think there were none.
|
||||
*
|
||||
* We can be wrong about the MOD if the file
|
||||
* has in fact been closed and re-opened.
|
||||
*
|
||||
* We can be wrong about the ADD if the
|
||||
* the fd has been re-created with a dup()
|
||||
* of the same file that it was before.
|
||||
*/
|
||||
op = EPOLL_CTL_MOD;
|
||||
}
|
||||
} else if ((ch->read_change & EV_CHANGE_DEL) ||
|
||||
(ch->write_change & EV_CHANGE_DEL)) {
|
||||
/* If we're deleting anything, we'll want to do a MOD
|
||||
* or a DEL. */
|
||||
op = EPOLL_CTL_DEL;
|
||||
|
||||
if (ch->read_change & EV_CHANGE_DEL) {
|
||||
if (ch->write_change & EV_CHANGE_DEL) {
|
||||
events = EPOLLIN|EPOLLOUT;
|
||||
} else if (ch->old_events & EV_WRITE) {
|
||||
events = EPOLLOUT;
|
||||
op = EPOLL_CTL_MOD;
|
||||
} else {
|
||||
events = EPOLLIN;
|
||||
}
|
||||
} else if (ch->write_change & EV_CHANGE_DEL) {
|
||||
if (ch->old_events & EV_READ) {
|
||||
events = EPOLLIN;
|
||||
op = EPOLL_CTL_MOD;
|
||||
} else {
|
||||
events = EPOLLOUT;
|
||||
}
|
||||
}
|
||||
if (!events) {
|
||||
EVUTIL_ASSERT(op == 0);
|
||||
return 0;
|
||||
}
|
||||
|
||||
if (!events)
|
||||
return 0;
|
||||
if ((ch->read_change|ch->write_change) & EV_CHANGE_ET)
|
||||
events |= EPOLLET;
|
||||
|
||||
memset(&epev, 0, sizeof(epev));
|
||||
epev.data.fd = ch->fd;
|
||||
|
||||
57
make_epoll_table.py
Executable file
57
make_epoll_table.py
Executable file
@ -0,0 +1,57 @@
|
||||
#!/usr/bin/python
|
||||
|
||||
def get(old,wc,rc):
|
||||
if ('xxx' in (rc, wc)):
|
||||
return "0",-1
|
||||
|
||||
if ('add' in (rc, wc)):
|
||||
events = []
|
||||
if rc == 'add' or (rc != 'del' and 'r' in old):
|
||||
events.append("EPOLLIN")
|
||||
if wc == 'add' or (wc != 'del' and 'w' in old):
|
||||
events.append("EPOLLOUT")
|
||||
|
||||
if old == "0":
|
||||
op = "EPOLL_CTL_ADD"
|
||||
else:
|
||||
op = "EPOLL_CTL_MOD"
|
||||
return "|".join(events), op
|
||||
|
||||
if ('del' in (rc, wc)):
|
||||
op = "EPOLL_CTL_DEL"
|
||||
if rc == 'del':
|
||||
if wc == 'del':
|
||||
events = "EPOLLIN|EPOLLOUT"
|
||||
elif 'w' in old:
|
||||
events = "EPOLLOUT"
|
||||
op = "EPOLL_CTL_MOD"
|
||||
else:
|
||||
events = "EPOLLIN"
|
||||
else:
|
||||
assert wc == 'del'
|
||||
if 'r' in old:
|
||||
events = "EPOLLIN"
|
||||
op = "EPOLL_CTL_MOD"
|
||||
else:
|
||||
events = "EPOLLOUT"
|
||||
return events, op
|
||||
|
||||
return 0, 0
|
||||
|
||||
|
||||
def fmt(op, ev, old, wc, rc):
|
||||
entry = "{ %s, %s },"%(op, ev)
|
||||
assert len(entry)<=36
|
||||
sp = " "*(36-len(entry))
|
||||
print "\t%s%s/* old=%2s, write:%3s, read:%3s */" % (
|
||||
entry, sp, old, wc, rc)
|
||||
|
||||
|
||||
for old in ('0','r','w','rw'):
|
||||
for wc in ('0', 'add', 'del', 'xxx'):
|
||||
for rc in ('0', 'add', 'del', 'xxx'):
|
||||
|
||||
op,ev = get(old,wc,rc)
|
||||
|
||||
fmt(op, ev, old, wc, rc)
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user