linux-kernel - Re: [performance bug] kernel building regression on 64 LCPUs machine

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Date:	Fri, 21 Jan 2011 08:23:53 +0100
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	"Alex,Shi" <alex.shi@...el.com>
Cc:	vgoyal@...hat.com, jaxboe@...ionio.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [performance bug] kernel building regression on 64 LCPUs machine

On Wed, Jan 19, 2011 at 2:55 AM, Alex,Shi <alex.shi@...el.com> wrote:
> Shaohua and I tested kernel building performance on latest kernel. and
> found it is drop about 15% on our 64 LCPUs NHM-EX machine on ext4 file
> system. We find this performance dropping is due to commit
> 749ef9f8423054e326f. If we revert this patch or just change the
> WRITE_SYNC back to WRITE in jbd2/commit.c file. the performance can be
> recovered.
>
> iostat report show with the commit, read request merge number increased
> and write request merge dropped. The total request size increased and
> queue length dropped. So we tested another patch: only change WRITE_SYNC
> to WRITE_SYNC_PLUG in jbd2/commit.c, but nothing effected.
>
> we didn't test deadline IO mode, just test cfq. seems insert write
> request into sync queue effect much read performance, but we don't know
> details. What's your comments of this?
>
> iostat of .37 kernel:
> rrqm/s   wrqm/s r/s     w/s     rMB/s  wMB/s avgrq-sz avgqu-sz   await  svctm  %util
> 22.5772 96.46 92.3742 14.747  1.0048  0.439474 34.8557 0.18078 3.8076 0.30447 2.94302
> iostat of commit reverted .37:
> 26.6223 80.21 107.875 6.03538 1.51415 0 41.3275 0.153385 3.80569 0.377231 3.22323

>From these numbers, it seems to me that with the patch reverted, the
write bandwidth is really low, and probably you are keeping most
written files in the buffer cache during the whole compile, while the
non-reverted kernel is making progress in writing out the files. So
the 'improved' read bandwidth is due to unfairness w.r.t. writes.
Does the result change if you add a final sync and time that as well,
in order to see the full time to make it on disk?

I think that in a more extreme test where you end up filling all the
buffer cache with written data, you will see much longer stalls with
the revert than without.

Thanks,
Corrado

>
> vmstat report show, read bandwidth dropping:
> vmstat of .37:
>  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
> 3.4 52.6 0.0 64303937.0 16466.7 121544.5 0.0 0.0 2102.7 1914.6 7414.1 3185.7 2.0 1.0 80.3 16.7 0.0
> vmstat of revert all from .37
> 2.2 35.8 0.0 64306767.4 17265.6 126101.2 0.0 0.0 2415.8 1619.1 8532.2 3556.2 2.5 1.1 83.0 13.3 0.0
>
> Regards
> Alex
>
> ===
> diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
> index 34a4861..27ac2f3 100644
> --- a/fs/jbd/commit.c
> +++ b/fs/jbd/commit.c
> @@ -294,7 +294,7 @@ void journal_commit_transaction(journal_t *journal)
>        int first_tag = 0;
>        int tag_flag;
>        int i;
> -       int write_op = WRITE_SYNC;
> +       int write_op = WRITE;
>
>        /*
>         * First job: lock down the current transaction and wait for
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index f3ad159..69ff08e 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -329,7 +329,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>        int tag_bytes = journal_tag_bytes(journal);
>        struct buffer_head *cbh = NULL; /* For transactional checksums */
>        __u32 crc32_sum = ~0;
> -       int write_op = WRITE_SYNC;
> +       int write_op = WRITE;
>
>        /*
>         * First job: lock down the current transaction and wait for
>
>
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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