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SubjectRe: 2.6.4-mm2
On 18 Mar, Andrew Morton wrote:
> 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?

No I haven't and I'm hesitant to. I have 52 single drives in one volume
and 14 single drives the other. Because it's PostgreSQL, my options are
to either run on a single drive (and not be able to drive the system as
hard) or to reconfigure my drives using hardware raid. I could do the
latter if you think it'll help. It's just a bit time consuming.

>> 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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