Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co

[Nick Piggin <npiggin@suse.de>]

On Fri, Jun 25, 2010 at 01:03:20PM +0200, Christoph Hellwig wrote:
> On Wed, Jun 23, 2010 at 09:44:20PM -0400, Vivek Goyal wrote:
> > Let me explain the general idling logic and then see if it makes sense in case
> > of WRITE_SYNC.
> > 
> > Once a request has completed, if the cfq queue is empty, we have two choices.
> > Either expire the cfq queue and move on to dispatch requests from a
> > different queue or we idle on the queue hoping we will get more IO from
> > same process/queue.
> 
> queues are basically processes in this context?
> 
> > Idling can help (on SATA disks with high seek cost), if
> > our guess was right and soon we got another request from same process. We
> > cut down on number of seeks hence increased throghput.
> 
> I don't really understand the logic behind this.  If we lots of I/O
> that actually is close to each other we should generally submit it in
> one batch.  That is true for pagecache writeback, that is true for
> metadata (at least in XFS..), and it's true for any sane application
> doing O_DIRECT / O_SYNC style I/O.
> 
> What workloads produde I/O that is local (not random) writes with small
> delays between the I/O requests?

Biggest thing is multiple small files operations like on the same
directory. Best case I measured back when doing AS io scheduler
versus deadline was about 100x improvement on a uncached kernel
grep workload when competing with a streaming writeout (the writeout
probably ended up going somewhat slower naturally, but it is fairer).