Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation

[Peter Zijlstra]

On Thu, Jul 03, 2014 at 03:39:11PM +1000, Dave Chinner wrote:
> There's another regression with the optimisitic spinning in rwsems
> as well: it increases the size of the struct rw_semaphore by 16
> bytes. That has increased the size of the struct xfs_inode by 32
> bytes.
> 
> That's pretty damn significant - it's no uncommon to see machines
> with tens of millions of cached XFS inodes, so increasing the size
> of the inode by 4% is actually very significant. That's enough to go
> from having a well balanced workload to not being able to fit the
> working set of inodes in memory.
> 
> Filesystem developers will do almost anything to remove a few bytes
> from the struct inode because inode cache footprint is extremely
> important for performance. We also tend to get upset and
> unreasonable when other people undo that hard work by making changes
> that bloat the generic structures embedded in the inode
> structures....

Jason Low actually did a patch, yesterday, to shrink rwsem back to its
old size (on 64bit).

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