Re: Patch nag: Bug #363400

[Hans Petter Jansson <hpj novell com>]

On Wed, 2007-03-28 at 15:12 +0200, Alexander Larsson wrote:
> > > On Wed, 2007-03-21 at 14:28 -0600, Hans Petter Jansson wrote:

> > > > http://bugzilla.gnome.org/show_bug.cgi?id=363400
> > > > 
> > > > Has a gnome-vfs patch that I believe to be correct. It works around a
> > > > serious performance issue on XFS file systems. It has been tested by at
> > > > least two users.

> But DROP_CACHE_BATCH_SIZE is far smaller (512k) than
> DROP_CACHE_SIZE_LIMIT (20 meg). Now, i didn't do much scientific
> research to get to the DROP_CACHE_SIZE_LIMIT value, but its a large
> change that might cause regressions (or be better, who really knows...)

My gut feeling is that it needs to be lower than 20MB, but this would be
very hard to measure because it's really only relevant when a) processes
are competing for cache or b) we are copying files that are being used
by other processes.

I basically arrived at the value based on what would be acceptable
fadvise() CPU overhead (<1%) for the XFS runs that were posted.

Will you accept the patch if I just set DROP_CACHE_BATCH_SIZE to 20MB?
It would preserve the lower fadvise() limit and cause cache "pollution"
of up to 20MB during the operation.

-- 
Hans Petter