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SubjectRe: 2.6.4-mm2
On 18 Mar, Jens Axboe wrote:
> On Thu, Mar 18 2004, Andrew Morton wrote:
>> > 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.
>
> Ugh that looks really bad, I wonder how it could possibly ever be this
> bad. Mark, please do do a run with 2.6.5-rc1-mm2, I'd very much like to
> see the profile there. If things get this bad, I need to think some more
> about how to best handle the 'when to invoke request_fn on unplug calls'
> logic again.
>
> Actually, please also do a run with 2.6.5-rc1-mm2 + inlined patch. For
> non-stacked dm on dm it should work and could make a lot of difference
> for you.
>
> --- drivers/block/ll_rw_blk.c~ 2004-03-18 20:26:17.088531084 +0100
> +++ drivers/block/ll_rw_blk.c 2004-03-18 20:26:44.773554953 +0100
> @@ -1134,11 +1134,8 @@
> if (test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags))
> return;
>
> - /*
> - * always call down, since we can race now with setting the plugged
> - * bit outside of the queue lock
> - */
> - blk_remove_plug(q);
> + if (!blk_remove_plug(q))
> + return;
>
> /*
> * was plugged, fire request_fn if queue has stuff to do
>

http://developer.osdl.org/markw/dbt2-pgsql/409/

There are the results with 2.6.5-rc1-mm2 with your patch with a 512kb
stripe width. The throughput didn't improve but it did cut the ticks
for dm_table_unplug_all in half. Should I continue with smaller stripe
widths?
-
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