linux-kernel - Re: [PATCH 2/2] /proc/stat: Add sysctl parameter to control irq counts latency

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Date:   Mon, 7 Jan 2019 11:19:14 -0500
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Miklos Szeredi <miklos@...redi.hu>,
        Daniel Colascione <dancol@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 2/2] /proc/stat: Add sysctl parameter to control irq
 counts latency

On 01/07/2019 11:14 AM, Matthew Wilcox wrote:
> On Mon, Jan 07, 2019 at 11:07:47AM -0500, Waiman Long wrote:
>>> Why are you caching the _output_ of calling sprintf(), rather than caching the
>>> values of each interrupt?
>>>
>> It is just faster to dump the whole string buffer than redoing the
>> number formatting each time when the values don't change. I can cache
>> the individual sums instead if it is the preferred by most.
> But it also consumes more memory.  Can you gather some measurements to
> find out what the performance difference is if you cache the values
> instead of the strings?

I allocate 11 bytes (10 digits + space) for each unsigned int IRQ value.
I needs 4 bytes for the unsigned value itself. So it is a saving of 7
bytes. With 4k irqs, it will be 28k.

I will run some measurements of caching the values versus saving the
string later this week.

Thanks,
Longman