lkml.org 
[lkml]   [2010]   [Apr]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure


Christian Ehrhardt wrote:
>
>
> Johannes Weiner wrote:
[...]

>>>
>>> It stays at ~85M with more writes which is approx 50% of my free 160M
>>> memory.
>>
>> Ok, so I am the idiot that got quoted on 'the active set is not too
>> big, so
>> buffer heads are not a problem when avoiding to scan it' in eternal
>> history.
>>
>> But the threshold inactive/active ratio for skipping active file pages is
>> actually 1:1.
>>
>> The easiest 'fix' is probably to change that ratio, 2:1 (or even 3:1?)
>> appears
>> to be a bit more natural anyway? Below is a patch that changes it to
>> 2:1.
>> Christian, can you check if it fixes your regression?
>
> I'll check it out.
> from the numbers I have up to now I know that the good->bad transition
> for my case is somewhere between 30M/60M e.g. first and second write.
> The ratio 2:1 will eat max 53M of my ~160M that gets split up.
>
> That means setting the ratio to 2:1 or whatever else might help or not,
> but eventually there is just another setting of workload vs. memory
> constraints that would still be affected. Still I guess 3:1 (and I'll
> try that as well) should be enough to be a bit more towards the save side.

For "my case" 2:1 is not enough, 3:1 almost and 4:1 fixes the issue.
Still as I mentioned before I think any value carved in stone can and
will be bad to some use case - as 1:1 is for mine.

If we end up being unable to fix it internally by allowing the system to
"forget" and eventually free old unused buffers at least somewhen - then
we should neither implement it as 2:1 nor 3:1 nor whatsoever, but as
userspace configurable e.g. /proc/sys/vm/active_inactive_ratio.

I hope your suggestion below or an extension to it will allow the kernel
to free the buffers somewhen. Depending on how good/fast this solution
then will work we can still modify the ratio if needed.

>> Additionally, we can always scan active file pages but only deactivate
>> them
>> when the ratio is off and otherwise strip buffers of clean pages.
>
> In think we need something that allows the system to forget its history
> somewhen - be it 1:1 or x:1 - if the workload changes "long enough"(tm)
> it should eventually throw all old things out.
> Like I described before many systems have different usage patterns when
> e.g. comparing day/night workload. So it is far from optimal if e.g. day
> write loads eat so much cache and never give it back for nightly huge
> reads tasks or something similar.
>
> Would your suggestion achieve that already?
> If not what kind change could?
>
[...]
--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2010-04-20 10:57    [W:0.093 / U:0.512 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site