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    SubjectRe: [performance bug] kernel building regression on 64 LCPUs machine
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    add Jan and Theodore to the loop.

    On Wed, 2011-01-19 at 09:55 +0800, Shi, Alex 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.
    since WRITE_SYNC_PLUG doesn't work, this isn't a simple no-write-merge issue.

    > 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
    >
    > 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
    >
    >




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