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

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Date:   Thu, 4 Jun 2020 09:38:32 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Andi Kleen <andi.kleen@...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

On Wed, Jun 03, 2020 at 07:28:53AM -0700, Andi Kleen wrote:
> > 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.

Agree. Maybe I should also put the time cost info into the code
comments in case someone noticed the slowdown.

Thanks,
Feng

> 
> -Andi