lkml.org 
[lkml]   [2010]   [Mar]   [15]   [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


Mel Gorman wrote:
> On Fri, Mar 12, 2010 at 09:37:55AM -0500, Andrew Morton wrote:
>> On Fri, 12 Mar 2010 13:15:05 +0100 Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> wrote:
>>
>>>> It still feels a bit unnatural though that the page allocator waits on
>>>> congestion when what it really cares about is watermarks. Even if this
>>>> patch works for Christian, I think it still has merit so will kick it a
>>>> few more times.
>>> In whatever way I can look at it watermark_wait should be supperior to
>>> congestion_wait. Because as Mel points out waiting for watermarks is
>>> what is semantically correct there.
>> If a direct-reclaimer waits for some thresholds to be achieved then what
>> task is doing reclaim?
>>
>> Ultimately, kswapd.
>
> Well, not quite. The direct reclaimer will still wake up after a timeout
> and try again regardless of whether watermarks have been met or not. The
> intention is to back after after direct reclaim has failed. Granted, the
> window during which a direct reclaim finishes and an allocation attempt
> occurs is unnecessarily large. This may be addressed by the patch that
> changes where cond_resched() is called.
>
>> This will introduce a hard dependency upon kswapd
>> activity. This might introduce scalability problems. And latency
>> problems if kswapd if off doodling with a slow device (say), or doing a
>> journal commit. And perhaps deadlocks if kswapd tries to take a lock
>> which one of the waiting-for-watermark direct relcaimers holds.
>>
>
> What lock could they be holding? Even if that is the case, the direct
> reclaimers do not wait indefinitily.
>
>> Generally, kswapd is an optional, best-effort latency optimisation
>> thing and we haven't designed for it to be a critical service.
>> Probably stuff would break were we to do so.
>>
>
> No disagreements there.
>
>> This is one of the reasons why we avoided creating such dependencies in
>> reclaim. Instead, what we do when a reclaimer is encountering lots of
>> dirty or in-flight pages is
>>
>> msleep(100);
>>
>> then try again. We're waiting for the disks, not kswapd.
>>
>> Only the hard-wired 100 is a bit silly, so we made the "100" variable,
>> inversely dependent upon the number of disks and their speed. If you
>> have more and faster disks then you sleep for less time.
>>
>> And that's what congestion_wait() does, in a very simplistic fashion.
>> It's a facility which direct-reclaimers use to ratelimit themselves in
>> inverse proportion to the speed with which the system can retire writes.
>>
>
> The problem being hit is when a direct reclaimer goes to sleep waiting
> on congestion when in reality there were not lots of dirty or in-flight
> pages. It goes to sleep for the wrong reasons and doesn't get woken up
> again until the timeout expires.
>
> Bear in mind that even if congestion clears, it just means that dirty
> pages are now clean although I admit that the next direct reclaim it
> does is going to encounter clean pages and should succeed.
>
> Lets see how the other patch that changes when cond_reched() gets called
> gets on. If it also works out, then it's harder to justify this patch.
> If it doesn't work out then it'll need to be kicked another few times.
>

Unfortunately "page-allocator: Attempt page allocation immediately after
direct reclaim" don't help. No improvement in the regression we had
fixed with the watermark wait patch.

-> *kick*^^


--

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: 2010-03-15 15:49    [W:0.094 / U:15.320 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site