Re: [PATCH 1/2] xfs: use rhashtable to track buffer cache

[Dave Chinner <david@fromorbit.com>]

On Tue, Oct 18, 2016 at 10:14:12PM +0200, Lucas Stach wrote:
> On filesystems with a lot of metadata and in metadata intensive workloads
> xfs_buf_find() is showing up at the top of the CPU cycles trace. Most of
> the CPU time is spent on CPU cache misses while traversing the rbtree.
> 
> As the buffer cache does not need any kind of ordering, but fast lookups
> a hashtable is the natural data structure to use. The rhashtable
> infrastructure provides a self-scaling hashtable implementation and
> allows lookups to proceed while the table is going through a resize
> operation.
> 
> This reduces the CPU-time spent for the lookups to 1/3 even for small
> filesystems with a relatively small number of cached buffers, with
> possibly much larger gains on higher loaded filesystems.
> 
> The minimum size of 4096 buckets was chosen as it was the size of the
> xfs buffer cache hash before it was converted to an rbtree.
> 
> Signed-off-by: Lucas Stach <dev@lynxeye.de>

This fails to compile on CONFIG_XFS_DEBUG=y kernels due to various
ASSERT statements not being updated appropriately. When making XFS
changes to core infrastructure like this, it is highly recommended
that you use CONFIG_XFS_DEBUG=y so that it catches mistakes and
assumptions that your changes violate at runtime....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com