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    SubjectRe: Dynamic configure max_cstate
    On Tue, Jul 28, 2009 at 12:11:35PM +0200, Andreas Mohr wrote:
    > As a very quick test, I tried a
    > while :; do :; done
    > loop in shell and renicing shell to 19 (to keep my CPU out of ACPI idle),
    > but bonnie -s 100 results initially looked promising yet turned out to
    > be inconsistent. The real way to test this would be idle=poll.
    > My test system was Athlon XP with /proc/acpi/processor/CPU0/power
    > latencies of 000 and 100 (the maximum allowed value, BTW) for C1/C2.

    OK, I just tested it properly.
    Rebooted, did 5 bonnie -s 100 with ACPI idle, rebooted and did another 5
    bonnie -s 100 with idle=poll, results:


    $ cat bonnie_ACPI_* /tmp/line bonnie_poll_*
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20084 95.3 19037 9.5 12286 4.7 18074 99.6 581752 96.6 28792.3 93.6
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 19235 93.5 24591 11.8 13916 4.3 17934 99.8 604429 100.3 27993.8 98.0
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 17221 86.3 30591 16.1 15404 5.4 18689 99.3 593296 92.7 28146.0 98.5
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20254 99.3 110095 55.9 15722 6.1 17901 99.5 601185 99.8 28675.5 100.4
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 18274 88.5 106909 53.2 10614 4.1 18759 99.7 598833 99.4 28461.6 92.5
    ========
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 15274 98.2 20206 9.7 17286 7.3 18055 99.4 608112 101.0 28424.0 99.5
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20545 99.1 25332 12.6 16392 6.1 17957 99.4 606706 100.7 27906.8 90.7
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20482 99.2 30907 13.6 17585 6.2 17867 99.1 608090 101.0 27919.1 97.7
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20863 99.4 138383 66.2 18945 7.6 17938 99.5 581421 96.5 27094.6 94.8
    -------Sequential Output-------- ---Sequential Input-- --Random--
    -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
    Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
    1* 100 20821 98.8 156821 70.4 11536 4.4 18747 99.0 603556 100.2 27677.8 96.9


    And these values (cumulative) result in:

    ACPI poll
    Per Char 95068 97985 +3.06%
    Block 291223 371649 +27.62%
    Rewrite 67942 81744 +20.31%
    Per Char 91357 90564 -0.87%
    Block 2979495 3007885 +0.95%
    RndSeek 142069.2 139022.3 -2.1%
    average: +8.16%

    Now the question is how much is due to idle state entry/exit latency
    and how much is due to ACPI idle/wakeup code path execution.

    Still, an average of +8.16% during 5 test runs each should be quite some incentive,
    and once there's a proper "idle latency skipping during expected I/O replies"
    even with idle/wakeup code path reinstated we should hopefully be able to keep
    some 5% improvement in disk access.

    Andreas Mohr


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