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OPTIM: polling: do not create update entries for FD removal

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In order to reduce the number of poller updates, we can benefit from
the fact that modern pollers use sampling to report readiness and that
under load they rarely report the same FD multiple times in a row. As
such it's not always necessary to disable such FDs especially when we're
almost certain they'll be re-enabled again and will require another set
of syscalls.

Now instead of creating an update for a (possibly temporary) removal,
we only perform this removal if the FD is reported again as ready while
inactive. In addition this is performed via another update so that
alternating workloads like transfers have a chance to re-enable the
FD without any syscall during the loop (typically after the data that
filled a buffer have been sent). However we only do that for single-
threaded FDs as the other ones require a more complex setup and are not
on the critical path.

This does cause a few spurious wakeups but almost totally eliminates the
calls to epoll_ctl() on connections seeing intermitent traffic like HTTP/1
to a server or client.

A typical example with 100k requests for 4 kB objects over 200 connections
shows that the number of epoll_ctl() calls doesn't depend on the number
of requests anymore but most exclusively on the number of established
connections:

Before:
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 57.09    0.499964           0    654361    321190 recvfrom
 38.33    0.335741           0    369097         1 epoll_wait
  4.56    0.039898           0     44643           epoll_ctl
  0.02    0.000211           1       200       200 connect
------ ----------- ----------- --------- --------- ----------------
100.00    0.875814               1068301    321391 total

After:
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 59.25    0.504676           0    657600    323630 recvfrom
 40.68    0.346560           0    374289         1 epoll_wait
  0.04    0.000370           0       620           epoll_ctl
  0.03    0.000228           1       200       200 connect
------ ----------- ----------- --------- --------- ----------------
100.00    0.851834               1032709    323831 total

As expected there is also a slight increase of epoll_wait() calls since
delaying de-activation of events can occasionally cause one spurious
wakeup.
This commit is contained in:
Willy Tarreau 2019-12-27 15:52:34 +01:00
parent 5d7dcc2a8e
commit eaf05be0ee
1 changed files with 26 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
include/proto/fd.h
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@ -197,7 +197,6 @@ static inline void fd_stop_recv(int fd)
if (!(fdtab[fd].state & FD_EV_ACTIVE_R) ||
!HA_ATOMIC_BTR(&fdtab[fd].state, FD_EV_ACTIVE_R_BIT))
return;
updt_fd_polling(fd);
}
/* Disable processing send events on fd <fd> */
@ -206,7 +205,6 @@ static inline void fd_stop_send(int fd)
if (!(fdtab[fd].state & FD_EV_ACTIVE_W) ||
!HA_ATOMIC_BTR(&fdtab[fd].state, FD_EV_ACTIVE_W_BIT))
return;
updt_fd_polling(fd);
}
/* Disable processing of events on fd <fd> for both directions. */
@ -220,7 +218,6 @@ static inline void fd_stop_both(int fd)
return;
new = old & ~FD_EV_ACTIVE_RW;
} while (unlikely(!_HA_ATOMIC_CAS(&fdtab[fd].state, &old, new)));
updt_fd_polling(fd);
}
/* Report that FD <fd> cannot receive anymore without polling (EAGAIN detected). */
@ -327,7 +324,7 @@ static inline void fd_update_events(int fd, unsigned char evts)
{
unsigned long locked = atleast2(fdtab[fd].thread_mask);
unsigned char old, new;
int new_flags;
int new_flags, must_stop;
new_flags =
((evts & FD_EV_READY_R) ? FD_POLL_IN : 0) |
@ -340,6 +337,21 @@ static inline void fd_update_events(int fd, unsigned char evts)
if ((fdtab[fd].ev & FD_EV_ACTIVE_W) && (evts & FD_EV_SHUT_W))
new_flags |= FD_POLL_ERR;
/* compute the inactive events reported late that must be stopped */
must_stop = 0;
if (unlikely(!fd_active(fd))) {
/* both sides stopped */
must_stop = FD_POLL_IN | FD_POLL_OUT;
}
else if (unlikely(!fd_recv_active(fd) && (evts & (FD_EV_READY_R | FD_EV_SHUT_R | FD_EV_ERR_R)))) {
/* only send remains */
must_stop = FD_POLL_IN;
}
else if (unlikely(!fd_send_active(fd) && (evts & (FD_EV_READY_W | FD_EV_SHUT_W | FD_EV_ERR_W)))) {
/* only recv remains */
must_stop = FD_POLL_OUT;
}
old = fdtab[fd].ev;
new = (old & FD_POLL_STICKY) | new_flags;
@ -361,6 +373,16 @@ static inline void fd_update_events(int fd, unsigned char evts)
if (fdtab[fd].iocb && fd_active(fd))
fdtab[fd].iocb(fd);
/* we had to stop this FD and it still must be stopped after the I/O
* cb's changes, so let's program an update for this.
*/
if (must_stop && !(fdtab[fd].update_mask & tid_bit)) {
if (((must_stop & FD_POLL_IN) && !fd_recv_active(fd)) ||
((must_stop & FD_POLL_OUT) && !fd_send_active(fd)))
if (!HA_ATOMIC_BTS(&fdtab[fd].update_mask, tid))
fd_updt[fd_nbupdt++] = fd;
}
ti->flags &= ~TI_FL_STUCK; // this thread is still running
}