linux-kernel - Re: [PATCH v4 2/4] mm/util.c: make vm_memory

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Date:   Wed, 3 Jun 2020 07:28:53 -0700
From:   Andi Kleen <andi.kleen@...el.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>, Qian Cai <cai@....pw>,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/4] mm/util.c: make vm_memory_committed() more
 accurate

> Its time cost is about 800 nanoseconds on a 2C/4T platform and
> 2~3 microseconds on a 2S/36C/72T server in normal case, and in
> worst case where vm_committed_as's spinlock is under severe
> contention, it costs 30~40 microseconds for the 2S/36C/72T sever,

This will be likely 40-80us on larger systems, although the overhead
is often non linear so it might get worse.

> which should be fine for its only two users: /proc/meminfo and
> HyperV balloon driver's status trace per second.

There are some setups who do frequent sampling of /proc/meminfo
in the background.  Increased overhead could be a problem for them.
But not proposing a change now. If someone complains have to 
revisit I guess, perhaps adding a rate limit of some sort.

-Andi