Xen devel - Oct 2007 - [PATCH] Dump flush write performance

Two changes to avoid disk driver using up kernel memory and triggering 
oom-killer
while creating guest dump to a disk file:

1) Decreased write chunks from 16MB to 2MB;
2) Flush (fsync) after each 2MB write.

To make sure we don''t suffer significant write performance penalty,
each
of the
following cases have been tested several times to dump a 1GB guest to a 
file on
a SAN disk:

With current code (16MB writes, sync once after all writes); ~38s
16MB writes, sync after each write: ~46s
2MB writes, sync after each write: ~49s

Sync-often costs us about 20%, smaller-write-size costs another 8%. The 
total is
less than 30% and about 10 seconds more for a 1GB guest.

Signed-off-by: Ben Guthro <bguthro@virtualron.com>
Signed-off-by: Daniel Li <dli@virtualiron.com>


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On 24/10/07 22:10, "Ben Guthro" <bguthro@virtualiron.com> wrote:
> Two changes to avoid disk driver using up kernel memory and triggering
> oom-killer
> while creating guest dump to a disk file:
> 
> 1) Decreased write chunks from 16MB to 2MB;
> 2) Flush (fsync) after each 2MB write.
I don''t think we''ve had any reports of core dumping causing
oom killer to
kick in. It seems a bit unlikely after all, unless you''re really
running
dom0 close to the wire. A user process write()ing a big file shouldn''t
be
able to trigger oom killer imo.

 -- Keir



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