Re: extra free kbytes tunable

[Rik van Riel <riel@redhat.com>]

On 02/15/2013 05:21 PM, Seiji Aguchi wrote:
> Rik, Satoru,
>
> Do you have any comments?

IIRC at the time the patch was rejected as too inelegant.

However, nobody else seems to have come up with a better plan, and
there are users in need of a fix for this problem.

I would still like to see a fix for the problem merged upstream.

>> -----Original Message-----
>> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of dormando
>> Sent: Monday, February 11, 2013 9:01 PM
>> To: Rik van Riel
>> Cc: Randy Dunlap; Satoru Moriya; linux-kernel@vger.kernel.org; linux-mm@kvack.org; lwoodman@redhat.com; Seiji Aguchi;
>> akpm@linux-foundation.org; hughd@google.com
>> Subject: extra free kbytes tunable
>>
>> Hi,
>>
>> As discussed in this thread:
>> http://marc.info/?l=linux-mm&m=131490523222031&w=2
>> (with this cleanup as well: https://lkml.org/lkml/2011/9/2/225)
>>
>> A tunable was proposed to allow specifying the distance between pages_min and the low watermark before kswapd is kicked in to
>> free up pages. I'd like to re-open this thread since the patch did not appear to go anywhere.
>>
>> We have a server workload wherein machines with 100G+ of "free" memory (used by page cache), scattered but frequent random io
>> reads from 12+ SSD's, and 5gbps+ of internet traffic, will frequently hit direct reclaim in a few different ways.
>>
>> 1) It'll run into small amounts of reclaim randomly (a few hundred thousand).
>>
>> 2) A burst of reads or traffic can cause extra pressure, which kswapd occasionally responds to by freeing up 40g+ of the pagecache all
>> at once
>> (!) while pausing the system (Argh).
>>
>> 3) A blip in an upstream provider or failover from a peer causes the kernel to allocate massive amounts of memory for retransmission
>> queues/etc, potentially along with buffered IO reads and (some, but not often a ton) of new allocations from an application. This
>> paired with 2) can cause the box to stall for 15+ seconds.
>>
>> We're seeing this more in 3.4/3.5/3.6, saw it less in 2.6.38. Mass reclaims are more common in newer kernels, but reclaims still happen
>> in all kernels without raising min_free_kbytes dramatically.
>>
>> I've found that setting "lowmem_reserve_ratio" to something like "1 1 32"
>> (thus protecting the DMA32 zone) causes 2) to happen less often, and is generally less violent with 1).
>>
>> Setting min_free_kbytes to 15G or more, paired with the above, has been the best at mitigating the issue. This is simply trying to raise
>> the distance between the min and low watermarks. With min_free_kbytes set to 15000000, that gives us a whopping 1.8G (!!!) of
>> leeway before slamming into direct reclaim.
>>
>> So, this patch is unfortunate but wonderful at letting us reclaim 10G+ of otherwise lost memory. Could we please revisit it?
>>
>> I saw a lot of discussion on doing this automatically, or making kswapd more efficient to it, and I'd love to do that. Beyond making
>> kswapd psychic I haven't seen any better options yet.
>>
>> The issue is more complex than simply having an application warn of an impending allocation, since this can happen via read load on
>> disk or from kernel page allocations for the network, or a combination of the two (or three, if you add the app back in).
>>
>> It's going to get worse as we push machines with faster SSD's and bigger networks. I'm open to any ideas on how to make kswapd
>> more efficient in our case, or really anything at all that works.
>>
>> I have more details, but cut it down as much as I could for this mail.
>>
>> Thanks,
>> -Dormando
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