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OPTIM: poll: restore polling after a poll/stop/want sequence

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If a file descriptor is being polled, and it stopped (eg: buffer full
or end of response), then re-enabled, currently what happens is that
the polling is disabled, then the fd is enabled in speculative mode,
an I/O attempt is made, it loses (otherwise the FD would surely not
have been polled), and the polled is enabled again.

This is too bad, especially with HTTP keep-alive on the server side
where all operations are performed at once before going back to the
poll loop.

Now we improve the behaviour by ensuring that if an fd is still being
polled, when it's enabled after having been disabled, we re-enable the
polling. Doing so saves a number of syscalls and useless wakeups, and
results in a significant performance gain on HTTP keep-alive. A 11%
increase has been observed on the HTTP request rate in keep-alive
thanks to this.

It could be considered as a bug fix, but there was no harm with the
current behaviour, except extra syscalls.
This commit is contained in:
Willy Tarreau 2013-12-27 20:11:50 +01:00
parent 72575509ca
commit 1208266356
1 changed files with 9 additions and 1 deletions
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10
include/proto/fd.h
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@ -171,7 +171,15 @@ static inline void fd_ev_set(int fd, int dir)
*/
if (i & (FD_EV_ACTIVE << dir))
return; /* already in desired state */
fdtab[fd].spec_e |= (FD_EV_ACTIVE << dir);
/* If we're touching an FD which is still being polled, and was
* recently disabled, we re-enable polling in order not to perform
* a syscall dance and to avoid a missed speculative event.
*/
if ((((unsigned int)fdtab[fd].spec_e) >> 4) & (FD_EV_POLLED << dir))
fdtab[fd].spec_e ^= i ^ (FD_EV_POLLED << dir);
else
fdtab[fd].spec_e |= (FD_EV_ACTIVE << dir);
updt_fd(fd); /* need an update entry to change the state */
}