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Replace big chain of if/thens in epoll.c with a table lookup
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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.
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@nmathewson
nmathewson committed Dec 16, 2010
1 parent b8b8aa5 commit 8c83eb6
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251 changes: 174 additions & 77 deletions epoll.c
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Original file line number Diff line number Diff line change
Expand Up @@ -167,94 +167,191 @@ 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,
const struct event_change *ch)
{
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!
*/

/* 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;
}
}
}
idx = INDEX(ch);
op = op_table[idx].op;
events = op_table[idx].events;

if (!events)
if (!events) {
EVUTIL_ASSERT(op == 0);
return 0;
}

if ((ch->read_change|ch->write_change) & EV_CHANGE_ET)
events |= EPOLLET;

memset(&epev, 0, sizeof(epev));
epev.data.fd = ch->fd;
Expand Down
57 changes: 57 additions & 0 deletions make_epoll_table.py
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Original file line number Diff line number Diff line change
@@ -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)

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