linux-kernel - Re: [PATCH 0/5] mm/memcg: Reduce kmemcache memory accounting overhead

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Date:   Mon, 12 Apr 2021 10:03:13 -0400
From:   Waiman Long <llong@...hat.com>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>
Subject: Re: [PATCH 0/5] mm/memcg: Reduce kmemcache memory accounting overhead

On 4/9/21 9:51 PM, Roman Gushchin wrote:
> On Fri, Apr 09, 2021 at 07:18:37PM -0400, Waiman Long wrote:
>> With the recent introduction of the new slab memory controller, we
>> eliminate the need for having separate kmemcaches for each memory
>> cgroup and reduce overall kernel memory usage. However, we also add
>> additional memory accounting overhead to each call of kmem_cache_alloc()
>> and kmem_cache_free().
>>
>> For workloads that require a lot of kmemcache allocations and
>> de-allocations, they may experience performance regression as illustrated
>> in [1].
>>
>> With a simple kernel module that performs repeated loop of 100,000,000
>> kmem_cache_alloc() and kmem_cache_free() of 64-byte object at module
>> init. The execution time to load the kernel module with and without
>> memory accounting were:
>>
>>    with accounting = 6.798s
>>    w/o  accounting = 1.758s
>>
>> That is an increase of 5.04s (287%). With this patchset applied, the
>> execution time became 4.254s. So the memory accounting overhead is now
>> 2.496s which is a 50% reduction.
> Hi Waiman!
>
> Thank you for working on it, it's indeed very useful!
> A couple of questions:
> 1) did your config included lockdep or not?
The test kernel is based on a production kernel config and so lockdep 
isn't enabled.
> 2) do you have a (rough) estimation how much each change contributes
>     to the overall reduction?

I should have a better breakdown of the effect of individual patches. I 
rerun the benchmarking module with turbo-boosting disabled to reduce 
run-to-run variation. The execution times were:

Before patch: time = 10.800s (with memory accounting), 2.848s (w/o 
accounting), overhead = 7.952s
After patch 2: time = 9.140s, overhead = 6.292s
After patch 3: time = 7.641s, overhead = 4.793s
After patch 5: time = 6.801s, overhead = 3.953s

Patches 1 & 4 are preparatory patches that should affect performance.

So the memory accounting overhead was reduced by about half.

Cheers,
Longman