linux-kernel - Re: read() syscall slowing down due to other threads?

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Message-ID: <CAPa8GCBdVXx+iF9QAoURWchAY0avesYNwbQs_Do1Kn6Pyx+8+Q@mail.gmail.com>
Date:	Tue, 1 May 2012 16:40:29 +1000
From:	Nick Piggin <npiggin@...il.com>
To:	George Porter <gmporter@...ucsd.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: read() syscall slowing down due to other threads?

On 1 May 2012 16:03, George Porter <gmporter@...ucsd.edu> wrote:
> Hi all,
>
> I've run into a weird phenomenon I was hoping someone could help me pin down.
>
> I've got a multithreaded program which reads data off of 8 input
> disks, and does some processing on it.  I have 8 Reader threads, each
> of which reads data off of one of the eight input disks.  That data
> gets passed to other threads, which do some processing (this is an 8
> core machine).  If I do no or minimal processing in those other
> threads, the read() calls go at the speed of the disks (~100 MBps).  I
> measure that speed by taking a timestamp before the read syscall, then
> afterwards, and dividing that into the read size (5MB).  All seems
> good.
>
> However, if I start doing more computation on those other threads, the
> read() syscalls take longer to read the same amount of data,
> eventually slowing down to 50 MBps (50% slower).  I've used
> setaffinity() to isolate the Reader threads to one set of cores, and
> the compute threads to a different set of cores, and so I don't think
> it is CPU/scheduling interference.
>
> Thoughts?  Has anyone run into this before?

It could be memory or coherency traffic that's causing a slowdown across
the system?

Could a dynamic core frequency / thermal throttling explain any of the
slowdown?
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