linux-kernel - Re: threads-max observe limits

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Message-ID: <20190924084854.GD23050@dhcp22.suse.cz>
Date:   Tue, 24 Sep 2019 10:48:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: threads-max observe limits

On Mon 23-09-19 16:23:40, Eric W. Biederman wrote:
> 
> Michal,
> 
> Thinking about this I have a hunch about what changed.  I think at some
> point we changed from 4k to 8k kernel stacks.  So I suspect if your
> client is seeing a lower threads-max it is because the size of the
> kernel data structures increased.

This is indeed the case. Starting since 6538b8ea886e ("x86_64: expand
kernel stack to 16K") (3.16) we use THREAD_SIZE_ORDER = 2 and that
halved the auto-tuned value.

In the particular case
3.12
kernel.threads-max = 515561

4.4
kernel.threads-max = 200000

Neither of the two values is really insane on 32GB machine. 

I am not sure we want/need to tune the max_thread value further. If
anything the tuning should be removed altogether if proven not useful in
general. But we definitely need a way to override this auto-tuning.

Thanks
-- 
Michal Hocko
SUSE Labs