From: Lee Revell <rlrevell@joe-job.com>
To: Gautam Thaker <gthaker@comcast.net>
Cc: linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>
Subject: Re: severe jitter experienced with "select()" in linux 2.6.14-rt22
Date: Thu, 15 Dec 2005 22:30:44 -0500 [thread overview]
Message-ID: <1134703845.12086.237.camel@mindpipe> (raw)
In-Reply-To: <43A21324.2050905@comcast.net>
On Thu, 2005-12-15 at 20:06 -0500, Gautam Thaker wrote:
>
> /proc/latency trace is full of lines such as these:
>
> <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
> <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
> <...>-3 0.... 20319us : __down_mutex (rt_run_flush)
>
> and
>
> "dmesg" says somethign like this:
>
> ( ubersock-4032 |#0): new 131 us user-latency.
> ( ubersock-4032 |#0): new 131 us user-latency.
> ( ubersock-4032 |#0): new 133 us user-latency.
> ( ubersock-4032 |#0): new 221 us user-latency.
> ( ubersock-4032 |#0): new 223 us user-latency.
> ( ubersock-4032 |#0): new 20629 us user-latency.
> root@blade8>
>
> When tracing I exit my test when a large latency is observed (in the
> case above a 20,629 usec value was observed by the "select()" test.
>
AI've seen this in my tests too, I think it's still a problem that
rt_run_flush can cause a 20ms+ non preemptible section.
Ingo mentioned that he may push softirq preemption upstream which would
fix this. You can also try tweaking these sysctls:
net.ipv4.route.gc_elasticity = 8
net.ipv4.route.gc_interval = 60
net.ipv4.route.gc_timeout = 300
net.ipv4.route.gc_min_interval_ms = 500
net.ipv4.route.gc_min_interval = 0
net.ipv4.route.gc_thresh = 4096
which AFAICT should let you tune the route cache garbage collection to
run more often and hopefully process fewer routes per run.
Lee
prev parent reply other threads:[~2005-12-16 3:27 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-12-16 1:06 severe jitter experienced with "select()" in linux 2.6.14-rt22 Gautam Thaker
2005-12-16 3:09 ` Steven Rostedt
2005-12-16 5:40 ` Gautam Thaker
2005-12-16 3:30 ` Lee Revell [this message]
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