performance degradation on kernel upgrade (due to commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f )

performance degradation on kernel upgrade (due to commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f )

[Jagdish Motwani]

Recently i upgraded my kernel from 2.6.29.6 to 2.6.35.14.

After upgrading i got very poor performance on my postgre database.


My test.sql contains 10000 postgre insert query


Linux  2.6.29.6

time psql -U user -d database -f test.sql  > /dev/null
real    0m 7.23s
user    0m 0.38s
sys     0m 0.12s



Linux  2.6.35.14

# time psql -U user -d database -f test.sql  > /dev/null
real    1m 4.05s
user    0m 0.44s
sys    0m 0.12

I even tried Linux 3.5.4 and got similar results.

Using git bisect, i got commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f

Is it a behavior change or am i missing something? Are there any 
workarounds for this?


Regards,
Jagdish Motwan

Re: performance degradation on kernel upgrade (due to commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f )

[Jeff Moyer]

Jagdish Motwani <jagdish.motwani@elitecore.com> writes:

> Recently i upgraded my kernel from 2.6.29.6 to 2.6.35.14.
>
> After upgrading i got very poor performance on my postgre database.
[...]
> Using git bisect, i got commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f
>
> Is it a behavior change or am i missing something? Are there any
> workarounds for this?

commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f
Author: Christoph Hellwig <hch@lst.de>
Date:   Thu Oct 29 14:14:04 2009 +0100

    blkdev: flush disk cache on ->fsync

This is a data integrity fix, and you really don't want to work around
it.

Cheers,
Jeff

Re: performance degradation on kernel upgrade (due to commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f)

[Andres Freund]

Hi,

On Friday, September 21, 2012 03:47:56 PM Jagdish Motwani wrote:
> Recently i upgraded my kernel from 2.6.29.6 to 2.6.35.14.
> 
> After upgrading i got very poor performance on my postgre database.
> 
> 
> My test.sql contains 10000 postgre insert query
> 
> 
> Linux  2.6.29.6
> 
> time psql -U user -d database -f test.sql  > /dev/null
> real    0m 7.23s
> user    0m 0.38s
> sys     0m 0.12s
> 
> 
> 
> Linux  2.6.35.14
> 
> # time psql -U user -d database -f test.sql  > /dev/null
> real    1m 4.05s
> user    0m 0.44s
> sys    0m 0.12

How do the results to this look if you use psql -1/--single-transaction?

> I even tried Linux 3.5.4 and got similar results.
> 
> Using git bisect, i got commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f
> 
> Is it a behavior change or am i missing something? Are there any
> workarounds for this?
I guess youre using some form of virtualization? I think what youre observing 
is just that access via raw devices previously lied about consistency. As the 
commit observes several virtualization solutions can use raw device access.

If all those 10000 inserts above happen in individual transactions - which 
would happen if youre not using transactions explicitly -  each and every one 
of them will cause a single disk write if they are executed sequentially.
A typical rotating disks can do between 80-160 such writes. If you devide 10k 
transactions by 150 synchronous writes a second you get ~66s which pretty 
nicely fits your time above.

If you don't care about loosing a very small amount of transactions (up to a 
second with the default settings) you can disable the synchronous_commit 
setting in postgres. No earlier commits/changes will be lost/corrupted.
You can change that setting per transaction, per session/connection, per user, 
per database or globally.

Greetings,

Andres

Re: performance degradation on kernel upgrade (due to commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f)

[Jagdish Motwani]

On 09/21/2012 07:55 PM, Andres Freund wrote:
> Hi,
>
> On Friday, September 21, 2012 03:47:56 PM Jagdish Motwani wrote:
>> Recently i upgraded my kernel from 2.6.29.6 to 2.6.35.14.
>>
>> After upgrading i got very poor performance on my postgre database.
>>
>>
>> My test.sql contains 10000 postgre insert query
>>
>>
>> Linux  2.6.29.6
>>
>> time psql -U user -d database -f test.sql  > /dev/null
>> real    0m 7.23s
>> user    0m 0.38s
>> sys     0m 0.12s
>>
>>
>>
>> Linux  2.6.35.14
>>
>> # time psql -U user -d database -f test.sql  > /dev/null
>> real    1m 4.05s
>> user    0m 0.44s
>> sys    0m 0.12
> How do the results to this look if you use psql -1/--single-transaction?
Using single transaction solves the problem - but that's not what i 
really want.
>
>> I even tried Linux 3.5.4 and got similar results.
>>
>> Using git bisect, i got commit ab0a9735e06914ce4d2a94ffa41497dbc142fe7f
>>
>> Is it a behavior change or am i missing something? Are there any
>> workarounds for this?
> I guess youre using some form of virtualization? I think what youre observing
> is just that access via raw devices previously lied about consistency. As the
> commit observes several virtualization solutions can use raw device access.
>
> If all those 10000 inserts above happen in individual transactions - which
> would happen if youre not using transactions explicitly -  each and every one
> of them will cause a single disk write if they are executed sequentially.
> A typical rotating disks can do between 80-160 such writes. If you devide 10k
> transactions by 150 synchronous writes a second you get ~66s which pretty
> nicely fits your time above.
>
> If you don't care about loosing a very small amount of transactions (up to a
> second with the default settings) you can disable the synchronous_commit
> setting in postgres. No earlier commits/changes will be lost/corrupted.
> You can change that setting per transaction, per session/connection, per user,
> per database or globally.
>
> Greetings,
>
> Andres
Thanks Andres,

I disabled synchronous_commit, and got results similar to 2.6.32.

Regards,
Jagdish Motwani