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

[Waiman Long]

On 4/12/21 1:47 PM, Roman Gushchin wrote:
> On Mon, Apr 12, 2021 at 10:03:13AM -0400, Waiman Long wrote:
> > 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
> Thank you! If there will be v2, I'd include this information into commit logs.

Yes, I am planning to send out v2 with these information in the 
cover-letter. I am just waiting a bit to see if there are more feedback.

-Longman

> > Patches 1 & 4 are preparatory patches that should affect performance.
> >
> > So the memory accounting overhead was reduced by about half.

BTW, the benchmark that I used is kind of the best case behavior as it 
as all updates are to the percpu stocks. Real workloads will likely to 
have a certain amount of update to the memcg charges and vmstats. So the 
performance benefit will be less.

Cheers,
Longman