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    SubjectRe: 2.6.4-mm2
    markw@osdl.org wrote:
    >
    > Sorry I'm falling behind... I see about a 10% decrease in throughput
    > with our dbt2 workload when comparing 2.6.4-mm2 to 2.6.3. I'm wondering
    > if this might be a result of the changes to the pagecache, radix-tree
    > and writeback code since you mentioned it could affect i/o scheduling in
    > 2.6.4-mm1.

    Could be. Have you run tests without LVM in the picture?

    > PostgreSQL is using 8KB blocks and the characteristics of the i/o should
    > be that one lvm2 volume is experiencing mostly sequential writes, while
    > another has random reading and writing. Both these volumes are using
    > ext2. I'll summarize the throughput results here, with the lvm2 stripe
    > width varying across the columns:
    >
    > kernel 16 kb 32 kb 64 kb 128 kb 256 kb 512 kb
    > 2.6.3 2308 2335 2348 2334
    > 2.6.4-mm2 2028 2048 2074 2096 2082 2078
    >
    > Here's a page with links to profile, oprofile, etc of each result:
    > http://developer.osdl.org/markw/linux/2.6-pagecache.html
    >
    > Comparing one pair of readprofile results, I find it curious that
    > dm_table_unplug_all and dm_table_any_congested show up near the top of a
    > 2.6.4-mm2 profile when they haven't shown up before in 2.6.3.

    14015190 poll_idle 241641.2069
    175162 generic_unplug_device 1317.0075
    165480 __copy_from_user_ll 1272.9231
    161151 __copy_to_user_ll 1342.9250
    152106 schedule 85.0705
    142395 DAC960_LP_InterruptHandler 761.4706
    113677 dm_table_unplug_all 1386.3049
    65420 __make_request 45.5571
    64832 dm_table_any_congested 697.1183
    37913 try_to_wake_up 32.2939

    That's broken. How many disks are involve in the DM stack?

    The relevant code was reworked subsequent to 2.6.4-mm2. Maybe we fixed
    this, but I cannot immediately explain what you're seeing here.

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