Re: [PATCH v3 0/5] mm/memcg: Reduce kmemcache memory accounting overhead

Masayoshi Mizuma
2021-04-15T03:26:50Z

On Tue, Apr 13, 2021 at 09:20:22PM -0400, Waiman Long wrote:
> v3:
> - Add missing "inline" qualifier to the alternate mod_obj_stock_state()
> in patch 3.
> - Remove redundant current_obj_stock() call in patch 5.
>
> v2:
> - Fix bug found by test robot in patch 5.
> - Update cover letter and commit logs.
>
> 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] and [2].
>
> A simple kernel module that performs repeated loop of 100,000,000
> kmem_cache_alloc() and kmem_cache_free() of a 64-byte object at module
> init time is used for benchmarking. The test was run on a CascadeLake
> server with turbo-boosting disable to reduce run-to-run variation.
>
> With memory accounting disable, the run time was 2.848s. With memory
> accounting enabled, the run times with the application of various
> patches in the patchset were:
>
> Applied patches Run time Accounting overhead Overhead %age
> --------------- -------- ------------------- -------------
> None 10.800s 7.952s 100.0%
> 1-2 9.140s 6.292s 79.1%
> 1-3 7.641s 4.793s 60.3%
> 1-5 6.801s 3.953s 49.7%
>
> Note that this is the best case scenario where most updates happen only
> to the percpu stocks. Real workloads will likely have a certain amount
> of updates to the memcg charges and vmstats. So the performance benefit
> will be less.
>
> It was found that a big part of the memory accounting overhead
> was caused by the local_irq_save()/local_irq_restore() sequences in
> updating local stock charge bytes and vmstat array, at least in x86
> systems. There are two such sequences in kmem_cache_alloc() and two
> in kmem_cache_free(). This patchset tries to reduce the use of such
> sequences as much as possible. In fact, it eliminates them in the common
> case. Another part of this patchset to cache the vmstat data update in
> the local stock as well which also helps.
>
> [1] https://lore.kernel.org/linux-mm/20210408193948.vfktg3azh2wrt56t@gabell/T/#u

Hi Longman,

Thank you for your patches.
I rerun the benchmark with your patches, it seems that the reduction
is small... The total duration of sendto() and recvfrom() system call
during the benchmark are as follows.

- sendto
- v5.8 vanilla: 2576.056 msec (100%)
- v5.12-rc7 vanilla: 2988.911 msec (116%)
- v5.12-rc7 with your patches (1-5): 2984.307 msec (115%)

- recvfrom
- v5.8 vanilla: 2113.156 msec (100%)
- v5.12-rc7 vanilla: 2305.810 msec (109%)
- v5.12-rc7 with your patches (1-5): 2287.351 msec (108%)

kmem_cache_alloc()/kmem_cache_free() are called around 1,400,000 times during
the benchmark. I ran a loop in a kernel module as following. The duration
is reduced by your patches actually.

---
dummy_cache = KMEM_CACHE(dummy, SLAB_ACCOUNT);
for (i = 0; i < 1400000; i++) {
p = kmem_cache_alloc(dummy_cache, GFP_KERNEL);
kmem_cache_free(dummy_cache, p);
}
---

- v5.12-rc7 vanilla: 110 msec (100%)
- v5.12-rc7 with your patches (1-5): 85 msec (77%)

It seems that the reduction is small for the benchmark though...
Anyway, I can see your patches reduce the overhead.
Please feel free to add:

Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>

Thanks!
Masa

> [2] https://lore.kernel.org/lkml/20210114025151.GA22932@xsang-OptiPlex-9020/
>
> Waiman Long (5):
> mm/memcg: Pass both memcg and lruvec to mod_memcg_lruvec_state()
> mm/memcg: Introduce obj_cgroup_uncharge_mod_state()
> mm/memcg: Cache vmstat data in percpu memcg_stock_pcp
> mm/memcg: Separate out object stock data into its own struct
> mm/memcg: Optimize user context object stock access
>
> include/linux/memcontrol.h | 14 ++-
> mm/memcontrol.c | 199 ++++++++++++++++++++++++++++++++-----
> mm/percpu.c | 9 +-
> mm/slab.h | 32 +++---
> 4 files changed, 196 insertions(+), 58 deletions(-)
>
> --
> 2.18.1
>