Xen devel - Feb 2007 - RE: [PATCH][TOOLS] Reducing impact ofdomainsave/restore/dump on Dom0

> > Reduce impact of saving/restoring/dumping large domains on Dom0
> memory
> > usage by means of fadvise64() to tell the OS to discard the cache
> pages
> > used for the save/dump file.
> >
> > Signed-off-by: Simon Graham <Simon.Graham@stratus.com>
> 
> Could the fadvise() logic be shared more than it is in this patch?
Also
> you
> sometimes sync-then-fadvise. Is this just a performance enhancement
> (presumably fadvise wouldn''t simply discard dirty pages in the
buffer
> cache)?
> 
1. You mean putting the fsync/fadvise in a common utility routine? I can
do that.

2. sync-then-advise is only done at the end of writing a file to ensure
that all
   of the cached pages are discarded. Whilst writing the file, I only
fadvise
   which triggers a write back and discards any clean pages up to the
specified offset.
   This is indeed a performance thing -- fsyncing on every write makes
it very slow.

Updated patch will follow...


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On 21/2/07 15:06, "Graham, Simon" <Simon.Graham@stratus.com>
wrote:
> 2. sync-then-advise is only done at the end of writing a file to ensure
> that all
>    of the cached pages are discarded. Whilst writing the file, I only
> fadvise
>    which triggers a write back and discards any clean pages up to the
> specified offset.
>    This is indeed a performance thing -- fsyncing on every write makes
> it very slow.
Do you need the fsync at all? It''s possible that the kernel will
launder-then-discard the affected pages automatically, just from the
fadvise() alone.

 -- Keir


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