Re: [PATCH v7.1] block: Coordinate flush requests

["Darrick J. Wong" <djwong@us.ibm.com>]

On Thu, Jan 13, 2011 at 01:38:55PM +0800, Shaohua Li wrote:
> 2011/1/13 Darrick J. Wong <djwong@us.ibm.com>:
> > On certain types of storage hardware, flushing the write cache takes a
> > considerable amount of time.  Typically, these are simple storage systems with
> > write cache enabled and no battery to save that cache during a power failure.
> > When we encounter a system with many I/O threads that try to flush the cache,
> > performance is suboptimal because each of those threads issues its own flush
> > command to the drive instead of trying to coordinate the flushes, thereby
> > wasting execution time.
> >
> > Instead of each thread initiating its own flush, we now try to detect the
> > situation where multiple threads are issuing flush requests.  The first thread
> > to enter blkdev_issue_flush becomes the owner of the flush, and all threads
> > that enter blkdev_issue_flush before the flush finishes are queued up to wait
> > for the next flush.  When that first flush finishes, one of those sleeping
> > threads is woken up to perform the next flush and then wake up the other
> > threads which are asleep waiting for the second flush to finish.
> >
> > In the single-threaded case, the thread will simply issue the flush and exit.
> >
> > To test the performance of this latest patch, I created a spreadsheet
> > reflecting the performance numbers I obtained with the same ffsb fsync-happy
> > workload that I've been running all along:  http://tinyurl.com/6xqk5bs
> >
> > The second tab of the workbook provides easy comparisons of the performance
> > before and after adding flush coordination to the block layer.  Variations in
> > the runs were never more than about 5%, so the slight performance increases and
> > decreases are negligible.  It is expected that devices with low flush times
> > should not show much change, whether the low flush times are due to the lack of
> > write cache or the controller having a battery and thereby ignoring the flush
> > command.
> >
> > Notice that the elm3b231_ipr, elm3b231_bigfc, elm3b57, elm3c44_ssd,
> > elm3c44_sata_wc, and elm3c71_scsi profiles showed large performance increases
> > from flush coordination.  These 6 configurations all feature large write caches
> > without battery backups, and fairly high (or at least non-zero) average flush
> > times, as was discovered when I studied the v6 patch.
> >
> > Unfortunately, there is one very odd regression: elm3c44_sas.  This profile is
> > a couple of battery-backed RAID cabinets striped together with raid0 on md.  I
> > suspect that there is some sort of problematic interaction with md, because
> > running ffsb on the individual hardware arrays produces numbers similar to
> > elm3c71_extsas.  elm3c71_extsas uses the same type of hardware array as does
> > elm3c44_sas, in fact.
> >
> > FYI, the flush coordination patch shows performance improvements both with and
> > without Christoph's patch that issues pure flushes directly.  The spreadsheet
> > only captures the performance numbers collected without Christoph's patch.
> Hi,
> can you explain why there is improvement with your patch? If there are
> multiple flush, blk_do_flush already has queue for them (the
> ->pending_flushes list).

With the current code, if we have n threads trying to issue flushes, the block
layer will issue n flushes one after the other.  I think the point of
Christoph's pure-flush patch is to skip the serialization step and allow
issuing of the n pure flushes in parallel.  The point of this patch is optimize
even more aggressively, such that as soon as the system becomes free to process
a pure flush (at time t), all the requests for a pure flush that were created
since the last time a pure flush was actually issued can be covered with a
single flush issued at time t.  In other words, this patch tries to reduce the
number of pure flushes issued.

--D
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