Re: sys_epoll_wait high CPU load in 2.6.37
From: Eric Dumazet
Date: Wed Jan 26 2011 - 06:16:31 EST
Le mercredi 26 janvier 2011 Ã 08:18 +0100, Eric Dumazet a Ãcrit :
> Le mardi 25 janvier 2011 Ã 16:09 -0800, Simon Kirby a Ãcrit :
> > Hello!
> >
> > Since upgrading 2.6.36 -> 2.6.37, dovecot's "anvil" process seems to end
> > up taking a lot more time in "top", and "perf top" shows output like this
> > (system-wide):
> >
> > samples pcnt function DSO
> > _______ _____ _____________________________ __________________________
> >
> > 2405.00 68.8% sys_epoll_wait [kernel.kallsyms]
> > 33.00 0.9% mail_cache_lookup_iter_next libdovecot-storage.so.0.0.0
> > 30.00 0.9% _raw_spin_lock [kernel.kallsyms]
> > ...etc...
> >
> > It only wakes up 5-10 times per second or so (on this box), and does
> > stuff like this:
> >
> > epoll_wait(12, {{EPOLLIN, {u32=19417616, u64=19417616}}}, 25, 2147483647) = 1
> > read(29, "PENALTY-GET\t192.168.31.10\n"..., 738) = 26
> > write(29, "0 0\n"..., 4) = 4
> > epoll_wait(12, {{EPOLLIN, {u32=19395632, u64=19395632}}}, 25, 2147483647) = 1
> > read(18, "LOOKUP\tpop3/192.168.31.10/tshield"..., 668) = 58
> > write(18, "0\n"..., 2) = 2
> > epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
> > read(7, "CONNECT\t3490\tpop3/192.168.31.10/t"..., 254) = 64
> > epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
> > read(7, "DISCONNECT\t3482\tpop3/192.168.31.1"..., 190) = 62
> >
> > Anything obvious here? anvil talks over UNIX sockets to the rest of
> > dovecot, and uses epoll_wait. So, suspect commits might be:
> >
> > 95aac7b1cd224f568fb83937044cd303ff11b029
> > 5456f09aaf88731e16dbcea7522cb330b6846415
> > or other bits from
> > git log v2.6.36..v2.6.37 net/unix/af_unix.c fs/eventpoll.c
> >
> > I suspect it has something to do with that "infinite value" check removal
> > in that first commit. It doesn't show up easily on a test box, but I can
> > try reverting 95aac7b1cd in production if it's not obvious.
> >
> > Simon-
>
> Yes, 95aac7b1cd is the problem, but anvil should use a 0 (no) timeout
> instead of 2147483647 ms : epoll_wait() doesnt have to arm a timer in
> this case, it is a bit faster.
>
>
Slowness comes from timespec_add_ns() : This one assumed small 'ns'
argument, since it wants to avoid a divide instruction.
static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
{
a->tv_sec += __iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
a->tv_nsec = ns;
}
We should do this differently for epoll usage ;)
Please try following patch :
[PATCH] epoll: epoll_wait() should be careful in timespec_add_ns use
commit 95aac7b1cd224f (epoll: make epoll_wait() use the hrtimer range
feature) added a performance regression because it used
timespec_add_ns() with potential very large 'ns' values.
Reported-by: Simon Kirby <sim@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
CC: Shawn Bohrer <shawn.bohrer@xxxxxxxxx>
CC: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/eventpoll.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cc8a9b7..7ec0890 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1126,7 +1126,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
if (timeout > 0) {
ktime_get_ts(&end_time);
- timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
+ end_time.tv_sec += timeout / MSEC_PER_SEC;
+ timeout %= MSEC_PER_SEC;
+ timespec_add_ns(&end_time, timeout * NSEC_PER_MSEC);
slack = select_estimate_accuracy(&end_time);
to = &expires;
*to = timespec_to_ktime(end_time);
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/