From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: readahead: Increase maximum readahead window
Date: Wed, 4 Oct 2017 10:41:51 -0700 [thread overview]
Message-ID: <20171004174151.GA6497@magnolia> (raw)
In-Reply-To: <20171004091205.468-1-jack@suse.cz>
On Wed, Oct 04, 2017 at 11:12:05AM +0200, Jan Kara wrote:
> Increase default maximum allowed readahead window from 128 KB to 512 KB.
> This improves performance for some workloads (see below for details) where
> ability to scale readahead window to larger sizes allows for better total
> throughput while chances for regression are rather low given readahead
> window size is dynamically computed based on observation (and thus it never
> grows large for workloads with a random read pattern).
>
> Note that the same tuning can be done using udev rules or by manually setting
> the sysctl parameter however we believe the new value is a better default most
> users will want to use. As a data point we carry this patch in SUSE kernels
> for over 8 years.
>
> Some data from the last evaluation of this patch (on 4.4-based kernel, I can
> rerun those tests on a newer kernel but nothing has changed in the readahead
> area since 4.4). The patch was evaluated on two machines
This is purely speculating, but I think this is worth at least a quick
retry on 4.14 to see what's changed in the past 10 kernel release. For
one thing, ext3 no longer exists, and XFS' file IO path has changed
quite a lot since then.
> o a UMA machine, 8 cores and rotary storage
> o A NUMA machine, 4 socket, 48 cores and SSD storage
>
> Five basic tests were conducted;
>
> 1. paralleldd-single
> paralleldd uses different instances of dd to access a single file and
> write the contents to /dev/null. The performance of it depends on how
> well readahead works for a single file. It's mostly sequential IO.
>
> 2. paralleldd-multi
> Similar to test 1 except each instance of dd accesses a different file
> so each instance of dd is accessing data sequentially but the timing
> makes it look like random read IO.
>
> 3. pgbench-small
> A standard init of pgbench and execution with a small data set
>
> 4. pgbench-large
> A standard init of pgbench and execution with a large data set
>
> 5. bonnie++ with dataset sizes 2X RAM and in asyncronous mode
>
> UMA paralleldd-single on ext3
> 4.4.0 4.4.0
> vanilla readahead-v1r1
> Amean Elapsd-1 5.42 ( 0.00%) 5.40 ( 0.50%)
> Amean Elapsd-3 7.51 ( 0.00%) 5.54 ( 26.25%)
> Amean Elapsd-5 7.15 ( 0.00%) 5.90 ( 17.46%)
> Amean Elapsd-7 5.81 ( 0.00%) 5.61 ( 3.42%)
> Amean Elapsd-8 6.05 ( 0.00%) 5.73 ( 5.36%)
>
> Results speak for themselves, readahead is a major boost when there
> are multiple readers of data. It's not displayed but system CPU
> usage is overall. The IO stats support the results
>
> 4.4.0 4.4.0
> vanillareadahead-v1r1
> Mean sda-avgqusz 7.44 8.59
> Mean sda-avgrqsz 279.77 722.52
> Mean sda-await 31.95 48.82
> Mean sda-r_await 3.32 11.58
> Mean sda-w_await 127.51 119.60
> Mean sda-svctm 1.47 3.46
> Mean sda-rrqm 27.82 23.52
> Mean sda-wrqm 4.52 5.00
>
> It shows that the average request size is 2.5 times larger even
> though the merging stats are similar. It's also interesting to
> note that average wait times are higher but more IO is being
> initiated per dd instance.
>
> It's interesting to note that this is specific to ext3 and that xfs showed
> a small regression with larger readahead.
>
> UMA paralleldd-single on xfs
> 4.4.0 4.4.0
> vanilla readahead-v1r1
> Min Elapsd-1 6.91 ( 0.00%) 7.10 ( -2.75%)
> Min Elapsd-3 6.77 ( 0.00%) 6.93 ( -2.36%)
> Min Elapsd-5 6.82 ( 0.00%) 7.00 ( -2.64%)
> Min Elapsd-7 6.84 ( 0.00%) 7.05 ( -3.07%)
> Min Elapsd-8 7.02 ( 0.00%) 7.04 ( -0.28%)
> Amean Elapsd-1 7.08 ( 0.00%) 7.20 ( -1.68%)
> Amean Elapsd-3 7.03 ( 0.00%) 7.12 ( -1.40%)
> Amean Elapsd-5 7.22 ( 0.00%) 7.38 ( -2.34%)
> Amean Elapsd-7 7.07 ( 0.00%) 7.19 ( -1.75%)
> Amean Elapsd-8 7.23 ( 0.00%) 7.23 ( -0.10%)
>
> The IO stats are not displayed but show a similar ratio to ext3 and system
> CPU usage is also lower. Hence, this slowdown is unexplained but may be
> due to differences in XFS in the read path and how it locks even though
> direct IO is not a factor. Tracing was not enabled to see what flags are
> passed into xfs_ilock to see if the IO is all behind one lock but it's
> one potential explanation.
>
> UMA paralleldd-single on ext3
>
> This showed nothing interesting as the test was too short-lived to draw
> any conclusions. There was some difference in the kernels but it was
> within the noise. The same applies for XFS.
>
> UMA pgbench-small on ext3
>
> This showed very little that was interesting. The database load time
> was slower but by a very small margin. The actual transaction times
> were highly variable and inconclusive.
>
> NUMA pgbench-small on ext3
>
> Load times are not reported but they completed 1.5% faster.
>
> 4.4.0 4.4.0
> vanilla readahead-v1r1
> Hmean 1 3000.54 ( 0.00%) 2895.28 ( -3.51%)
> Hmean 8 20596.33 ( 0.00%) 19291.92 ( -6.33%)
> Hmean 12 30760.68 ( 0.00%) 30019.58 ( -2.41%)
> Hmean 24 74383.22 ( 0.00%) 73580.80 ( -1.08%)
> Hmean 32 88377.30 ( 0.00%) 88928.70 ( 0.62%)
> Hmean 48 88133.53 ( 0.00%) 96099.16 ( 9.04%)
> Hmean 80 55981.37 ( 0.00%) 76886.10 ( 37.34%)
> Hmean 112 74060.29 ( 0.00%) 87632.95 ( 18.33%)
> Hmean 144 51331.50 ( 0.00%) 66135.77 ( 28.84%)
> Hmean 172 44256.92 ( 0.00%) 63521.73 ( 43.53%)
> Hmean 192 35942.74 ( 0.00%) 71121.35 ( 97.87%)
>
> The impact here is substantial particularly for higher thread-counts.
> It's interesting to note that there is an apparent regression for low
> thread counts. In general, there was a high degree of variability
> but the gains were all outside of the noise. In general, the io stats
> did not show any particular pattern about request size as the workload
> is mostly resident in memory. The real curiousity is that readahead
> should have had little or no impact here as the data is mostly resident
> in memory. Observing the transactions over time, there was a lot of
> variability and the performance is likely dominated by whether the
> data happened to be local or not. In itself, this test does not push
> for inclusion of the patch due to the lack of IO but is included for
> completeness.
>
> UMA pgbench-small on xfs
>
> Similar observations to ext3 on the load times. The transaction times
> were stable but showed no significant performance difference.
>
> UMA pgbench-large on ext3
>
> Database load times were slightly faster (3.36%). The transaction times
> were slower on average, more variable but still very close to the noise.
>
> UMA pgbench-large on xfs
>
> No significant difference on either database load times or transactions.
>
> UMA bonnie on ext3
>
> 4.4.0 4.4.0
> vanilla readahead-v1r1
> Hmean SeqOut Char 81079.98 ( 0.00%) 81172.05 ( 0.11%)
> Hmean SeqOut Block 104416.12 ( 0.00%) 104116.24 ( -0.29%)
> Hmean SeqOut Rewrite 44153.34 ( 0.00%) 44596.23 ( 1.00%)
> Hmean SeqIn Char 88144.56 ( 0.00%) 91702.67 ( 4.04%)
> Hmean SeqIn Block 134581.06 ( 0.00%) 137245.71 ( 1.98%)
> Hmean Random seeks 258.46 ( 0.00%) 280.82 ( 8.65%)
> Hmean SeqCreate ops 2.25 ( 0.00%) 2.25 ( 0.00%)
> Hmean SeqCreate read 2.25 ( 0.00%) 2.25 ( 0.00%)
> Hmean SeqCreate del 911.29 ( 0.00%) 880.24 ( -3.41%)
> Hmean RandCreate ops 2.25 ( 0.00%) 2.25 ( 0.00%)
> Hmean RandCreate read 2.00 ( 0.00%) 2.25 ( 12.50%)
> Hmean RandCreate del 911.89 ( 0.00%) 878.80 ( -3.63%)
>
> The difference in headline performance figures is marginal and well within noise.
> The system CPU usage tells a slightly different story
>
> 4.4.0 4.4.0
> vanillareadahead-v1r1
> User 1817.53 1798.89
> System 499.40 420.65
> Elapsed 10692.67 10588.08
>
> As do the IO stats
>
> 4.4.0 4.4.0
> vanillareadahead-v1r1
> Mean sda-avgqusz 1079.16 1083.35
> Mean sda-avgrqsz 807.95 1225.08
> Mean sda-await 7308.06 9647.13
> Mean sda-r_await 119.04 133.27
> Mean sda-w_await 19106.20 20255.41
> Mean sda-svctm 4.67 7.02
> Mean sda-rrqm 1.80 0.99
> Mean sda-wrqm 5597.12 5723.32
>
> NUMA bonnie on ext3
>
> bonnie
> 4.4.0 4.4.0
> vanilla readahead-v1r1
> Hmean SeqOut Char 58660.72 ( 0.00%) 58930.39 ( 0.46%)
> Hmean SeqOut Block 253950.92 ( 0.00%) 261466.37 ( 2.96%)
> Hmean SeqOut Rewrite 151960.60 ( 0.00%) 161300.48 ( 6.15%)
> Hmean SeqIn Char 57015.41 ( 0.00%) 55699.16 ( -2.31%)
> Hmean SeqIn Block 600448.14 ( 0.00%) 627565.09 ( 4.52%)
> Hmean Random seeks 0.00 ( 0.00%) 0.00 ( 0.00%)
> Hmean SeqCreate ops 1.00 ( 0.00%) 1.00 ( 0.00%)
> Hmean SeqCreate read 3.00 ( 0.00%) 3.00 ( 0.00%)
> Hmean SeqCreate del 90.91 ( 0.00%) 79.88 (-12.14%)
> Hmean RandCreate ops 1.00 ( 0.00%) 1.50 ( 50.00%)
> Hmean RandCreate read 3.00 ( 0.00%) 3.00 ( 0.00%)
> Hmean RandCreate del 92.95 ( 0.00%) 93.97 ( 1.10%)
>
> The impact is small but in line with the UMA machine in a number of details.
> As before, the CPU usage is lower even if the iostats show very little
> differences overall.
>
> Overall, the headline performance figures are mostly improved or show
> little difference. There is a small anomaly with XFS that indicates it may
> not always win there due to other factors. There is also the possibility
/me wonders what the anomaly is/was?
(Well, not that much. If it disappears on 4.14 then I don't care at
all. :P)
--D
> that a mostly random read workload that was larger than memory with each
> read spanning multiple pages but less than the max readahead window would
> suffer but the probability is low as the readahead window should scale
> properly. On balance, this is a win -- particularly on the large read
> workloads.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> include/linux/mm.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00bad7793788..c50c6f442786 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1991,7 +1991,7 @@ int write_one_page(struct page *page, int wait);
> void task_dirty_inc(struct task_struct *tsk);
>
> /* readahead.c */
> -#define VM_MAX_READAHEAD 128 /* kbytes */
> +#define VM_MAX_READAHEAD 512 /* kbytes */
> #define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
>
> int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-10-04 17:41 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-04 9:12 [PATCH] mm: readahead: Increase maximum readahead window Jan Kara
2017-10-04 9:12 ` Jan Kara
2017-10-04 17:41 ` Darrick J. Wong [this message]
2017-10-05 8:39 ` Jan Kara
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20171004174151.GA6497@magnolia \
--to=darrick.wong@oracle.com \
--cc=akpm@linux-foundation.org \
--cc=jack@suse.cz \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-mm@kvack.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.