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

[Roman Gushchin]

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?
2) do you have a (rough) estimation how much each change contributes
   to the overall reduction?

Thanks!

> 
> It was found that a major part of the memory accounting overhead
> is 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
> 
> 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            | 198 ++++++++++++++++++++++++++++++++-----
>  mm/percpu.c                |   9 +-
>  mm/slab.h                  |  32 +++---
>  4 files changed, 195 insertions(+), 58 deletions(-)
> 
> -- 
> 2.18.1
>