On Mon, Nov 26, 2012 at 05:09:08PM -0500, Christoph Hellwig wrote:

On Mon, Nov 26, 2012 at 04:49:37PM -0500, Theodore Ts'o wrote:

quoted

Christoph, can you give some kind of estimate for the overhead that
adding this locking in XFS actually costs in practice?

I don't know any real life measurements, but in terms of implementation
the over head is:

 a) taking a the rw_semaphore in shared mode for every buffered read
 b) taking the slightly slower exclusive rw_semaphore for buffered writes
    instead of the plain mutex

On the other hand it significantly simplifies the locking for direct
I/O and allows parallel direct I/O writers.

I should probably just look at the XFS code, but.... if you're taking
an exclusve lock for buffered writes, won't this impact the
performance of buffered writes happening in parallel on different
CPU's?

						- Ted

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