linux-kernel - Re: sys_epoll_wait high CPU load in 2.6.37

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Date:	Wed, 26 Jan 2011 08:18:18 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Simon Kirby <sim@...tway.ca>
Cc:	linux-kernel@...r.kernel.org,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Davide Libenzi <davidel@...ilserver.org>
Subject: Re: sys_epoll_wait high CPU load in 2.6.37

Le mardi 25 janvier 2011 à 16:09 -0800, Simon Kirby a écrit :
> 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.



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