linux-kernel - Re: latencies due to disk writes

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Message-ID: <45E4E953.7060501@tmr.com>
Date:	Tue, 27 Feb 2007 21:30:43 -0500
From:	Bill Davidsen <davidsen@....com>
To:	kalium@....de
CC:	linux-kernel@...r.kernel.org
Subject: Re: latencies due to disk writes

kalium@....de wrote:
> Hello!
> 
> I'm experiencing extreme lags during disk writes. I have read somewhere (didn't save the URI, sigh) that this is actually related to bad (non-existing) write io priorities (CFQ only manages file reads).
> 
> I could imagine two quick, easy and probably quite effective ways to prevent such lags:
> 
> 1.) don't flush buffers to disk at once more than necessary.
> 
Actually, in many cases this is just what you do want, to avoid filling 
memory with buffered writes and then flushing them on time or memory runout.

Investigate the /proc/sys/vm/dirty_* values.

> 2.) relate CPU niceness to max write buffer fill level (ie. the point where it gets forced to be flushed to disk -- a conservative estimate would be much better than nothing): (100-5*nicelevel)%, ie. writes for processes having nice level 19 are blocked/delayed until the write buffer is below 5%. That way, the accounting is done at a higher and probably easier to access level.
> 
> Maybe I'm just talking nonsense, but nonetheless, here are my 2 cents.
> 
> Best regards,
> Mark
> 
> p.s. please CC me as I'm not subscribed to this list.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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