Date:	Thu, 13 Jan 2011 13:38:55 +0800
From:	Shaohua Li <shli@...nel.org>
To:	djwong@...ibm.com
Cc:	Jens Axboe <axboe@...nel.dk>, "Theodore Ts'o" <tytso@....edu>,
	Neil Brown <neilb@...e.de>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.cz>, Mike Snitzer <snitzer@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Keith Mannthey <kmannth@...ibm.com>,
	Mingming Cao <cmm@...ibm.com>, Tejun Heo <tj@...nel.org>,
	linux-ext4@...r.kernel.org, Ric Wheeler <rwheeler@...hat.com>,
	Christoph Hellwig <hch@....de>, Josef Bacik <josef@...hat.com>
Subject: Re: [PATCH v7.1] block: Coordinate flush requests

2011/1/13 Darrick J. Wong <djwong@...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).

Thanks,
Shaohua
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