Re: [PATCH v2 1/7] memcg: Scale up vmstats flush threshold with int_sqrt(nr_cpus+2)

Waiman Long
2026-03-25T16:47:42Z

On 3/23/26 8:15 PM, Yosry Ahmed wrote:
> On Mon, Mar 23, 2026 at 5:46 AM Li Wang <liwang@redhat.com> wrote:
>> On Fri, Mar 20, 2026 at 04:42:35PM -0400, Waiman Long wrote:
>>> The vmstats flush threshold currently increases linearly with the
>>> number of online CPUs. As the number of CPUs increases over time, it
>>> will become increasingly difficult to meet the threshold and update the
>>> vmstats data in a timely manner. These days, systems with hundreds of
>>> CPUs or even thousands of them are becoming more common.
>>>
>>> For example, the test_memcg_sock test of test_memcontrol always fails
>>> when running on an arm64 system with 128 CPUs. It is because the
>>> threshold is now 64*128 = 8192. With 4k page size, it needs changes in
>>> 32 MB of memory. It will be even worse with larger page size like 64k.
>>>
>>> To make the output of memory.stat more correct, it is better to scale
>>> up the threshold slower than linearly with the number of CPUs. The
>>> int_sqrt() function is a good compromise as suggested by Li Wang [1].
>>> An extra 2 is added to make sure that we will double the threshold for
>>> a 2-core system. The increase will be slower after that.
>>>
>>> With the int_sqrt() scale, we can use the possibly larger
>>> num_possible_cpus() instead of num_online_cpus() which may change at
>>> run time.
>>>
>>> Although there is supposed to be a periodic and asynchronous flush of
>>> vmstats every 2 seconds, the actual time lag between succesive runs
>>> can actually vary quite a bit. In fact, I have seen time lags of up
>>> to 10s of seconds in some cases. So we couldn't too rely on the hope
>>> that there will be an asynchronous vmstats flush every 2 seconds. This
>>> may be something we need to look into.
>>>
>>> [1] https://lore.kernel.org/lkml/ab0kAE7mJkEL9kWb@redhat.com/
>>>
>>> Suggested-by: Li Wang <liwang@redhat.com>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
> What's the motivation for this fix? Is it purely to make tests more
> reliable on systems with larger page sizes?
>
> We need some performance tests to make sure we're not flushing too
> eagerly with the sqrt scale imo. We need to make sure that when we
> have a lot of cgroups and a lot of flushers we don't end up performing
> worse.

I will include some performance data in the next version. Do you have
any suggestion of which readily available tests that I can use for this
performance testing purpose.

Cheers,
Longman