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Subject: Re: [PATCH 2/2] /proc/stat: Add sysctl parameter to control irq
 counts latency
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Daniel Colascione <dancol@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Randy Dunlap <rdunlap@infradead.org>
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From:   Waiman Long <longman@redhat.com>
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Message-ID: <f8eefa83-f85e-0ba8-dcb9-a76a12449228@redhat.com>
Date:   Mon, 7 Jan 2019 11:19:14 -0500
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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