Re: Bad SSD performance with recent kernels

[Shaohua Li]

On Tue, 2012-01-31 at 09:07 +0800, Wu Fengguang wrote:
> On Tue, Jan 31, 2012 at 08:14:19AM +0800, Li, Shaohua wrote:
> > On Mon, 2012-01-30 at 17:26 -0500, Vivek Goyal wrote:
> > > On Mon, Jan 30, 2012 at 03:51:49PM +0100, Eric Dumazet wrote:
> > > > Le lundi 30 janvier 2012 Ã 22:28 +0800, Wu Fengguang a Ãcrit :
> > > > > On Mon, Jan 30, 2012 at 06:31:34PM +0800, Li, Shaohua wrote:
> > > > > 
> > > > > > Looks the 2.6.39 block plug introduces some latency here. deleting
> > > > > > blk_start_plug/blk_finish_plug in generic_file_aio_read seems
> > > > > > workaround
> > > > > > the issue. The plug seems not good for sequential IO, because readahead
> > > > > > code already has plug and has fine grained control.
> > > > > 
> > > > > Why not remove the generic_file_aio_read() plug completely? It
> > > > > actually prevents unplugging immediately after the readahead IO is
> > > > > submitted and in turn stalls the IO pipeline as showed by Eric's
> > > > > blktrace data.
> > > > > 
> > > > > Eric, will you test this patch? Thank you.
> > > 
> > > Can you please run the blktrace again with this patch applied. I am curious
> > > to see how does traffic pattern look like now.
> > > 
> > > In your previous trace, there were so many small 8 sector requests which
> > > were merged into 512 sector requests before dispatching to disk. (I am
> > > not sure why those requests are not bigger. Shouldn't readahead logic
> > > submit a bigger request?) Now with plug/unplug logic removed, I am assuming
> > > we should be doing less merging and dispatching more smaller requests. May be
> > > that is helping and cutting down on disk idling time.
> > > 
> > > In previous logs, 512 sector request seems to be taking around 1ms to
> > > complete after dispatch. In between requests disk seems to be idle
> > > for around .5 to .6 ms. Out of this .3 ms seems to be gone in just
> > > coming up with new request after completion of previous one and another
> > > .3ms seems to be consumed in merging the smaller IOs. So if we don't wait
> > > for merging, it should keep disk busier for .3ms more which is 30% of time
> > > it takes to complete 512 sector request. So theoritically it can give
> > > 30% boost for this workload. (Assuming request size will not impact the
> > > disk throughput very severely).
> > > 
> > > Anyway, some blktrace data will shed some light..
> > yep, I suspect plug merges big request too (iostat shows it too), that's
> > why I only think delete the plug in generic_file_aio_read as a
> > workaround.
> 
> It's good to merge requests inside the same readahead window. However
> I don't think readahead window A should be merged with B at the cost
> of delaying A for some time, which will break the pipeline. If larger
> IO is desirable, we can do so by increasing the readahead size.
> 
> > I still thought readahead has something to do here. I
> > observed the async readahead does readahead (A, A + 2M), and follows (A
> > +128k, A+2M), (A+256k, A+2M) ..., the later readahead doesn't work
> > because we already have (A, A+2M) in memory at that time. Anyway, I can
> > reproduce the issue, will play with it more today.
> 
> How do you observe that? I don't think that readahead pattern is
> possible. However I do see such _read_ patterns.
Ok, after double checking the code and do some tracing, I'm now thinking
we should delete the plug code in generic_file_aio_read. I thought the
problem is:

T1: ra (A, A+128k), (A+128k, A+256k), submit the 256k because lock_page
T2: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
because of plug and there isn't any lock_page till we hit page A+256k
because all pages from A to A+ 256k is in memory
T3: hit page A+256k, ra (A+384, A+ 512). because plug, the range isn't
submitted again.
T4: lock_page A+256, so (A+256, A+512) will be submitted. The task is
waitting for (A+256, A+512) finish

so the pipeline doesn't work.

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