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To: Gregory Stark <gsstark@MIT.EDU>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Problem with read blocking for a long time on /dev/scd1
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From: Greg Stark <gsstark@mit.edu>
Organization: The Emacs Conspiracy; member since 1992
Date: 21 Dec 2002 01:14:38 -0500
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Gregory Stark <gsstark@MIT.EDU> writes:

> I'm having a problem with ogle that seems to be being caused by the scsi or
> ide-scsi driver. The video playback freezes for a second or randomly,
> sometimes every few seconds, sometimes not for several minutes. Every such
> glitch is correlated perfectly with a read syscall reading on /dev/scd1
> blocking for an inordinate amount of time.
> 
> Most read syscalls from ogle seem to take between 30us to 100ms depending on
> the size of the read. In fact plotting the time taken reported by strace -T vs
> the size of the read in gnuplot produces a nice obvious linear correlation.

One more piece of data I just found. Whenever the read blocks for a long time
like this it seems ps lists the process's wait channel as "lock_p". The only
matching symbol is lock_page. 

Does that help narrow down the source of the latency or is that just what the
device driver happens to use as its synchronization primitive?

--
greg

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