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Subject: Re: Strange load spikes on 2.4.19 kernel
Date: Sat, 12 Oct 2002 16:52:31 +1000
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> It commits your changes to the journal every five seconds.  But your data
> is then only in the journal.  It still needs to be written into your
files.
> That writeback is controlled by the normal kernel 30-second writeback
> timing.  If that writeback isn't keeping up, kjournald needs to
> force the writeback so it can recycle that data's space in the journal.
>
> While that writeback is happening, everything tends to wait on it.

Doesn't bdflush let you control this? I noted in my first post that we'd
played with changing bdflush params as described here:

http://www-106.ibm.com/developerworks/linux/library/l-fs8.html?dwzone=linux

And set them to this:

[root@server5 hm]# cat /proc/sys/vm/bdflush
39      500     0       0       60      300     60      0       0

Shouldn't this reduce it to writing every 3 seconds? We tried lowering some
of the values even further based on the description here:

http://www.bb-zone.com/zope/bbzone/docs/slgfg/part2/cha04/sec04

So we altered the first one (nfract) to 10(%) to try and keep the dirty
buffer list small, but that didn't help either. I sort of thought that
age_buffer at 3 seconds would be more likely to activate anyway than 40% of
buffers being dirty?

> It is suspected that ext3 gets the flushtime on those buffers
> wrong as well, so the writeback isn't happening right.

So you're saying that ext3 is somehow breaking the standard kernel writeback
code? Is this something they know about, and/or are addressing?

Rob

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