[Cluster-devel] [GFS2 PATCH] GFS2: Don't brelse rgrp buffer_heads every allocation

[Bob Peterson <rpeterso@redhat.com>]

----- Original Message -----
> Hi,
> 
> 
> On 05/06/15 15:49, Bob Peterson wrote:
> > Hi,
> >
> > This patch allows the block allocation code to retain the buffers
> > for the resource groups so they don't need to be re-read from buffer
> > cache with every request. This is a performance improvement that's
> > especially noticeable when resource groups are very large. For
> > example, with 2GB resource groups and 4K blocks, there can be 33
> > blocks for every resource group. This patch allows those 33 buffers
> > to be kept around and not read in and thrown away with every
> > operation. The buffers are released when the resource group is
> > either synced or invalidated.
> The blocks should be cached between operations, so this should only be
> resulting in a skip of the look up of the cached block, and no changes
> to the actual I/O. Does that mean that grab_cache_page() is slow I
> wonder? Or is this an issue of going around the retry loop due to lack
> of memory at some stage?
> 
> How does this interact with the rgrplvb support? I'd guess that with
> that turned on, this is no longer an issue, because we'd only read in
> the blocks for the rgrps that we are actually going to use?
> 
> 
> 
> Steve.

Hi,

If you compare the two vmstat outputs in the bugzilla #1154782, you'll
see no significant difference in memory usage nor cpu usage. So I assume
the page lookup is the "slow" part; not because it's such a slow thing
but because it's done 33 times per read-reference-invalidate (33 pages
to look up per rgrp).

Regards,

Bob Peterson
Red Hat File Systems