Re: [PATCH] proc: speedup /proc/stat handling

[Glauber Costa]

On 01/23/2012 02:16 PM, KAMEZAWA Hiroyuki wrote:
> On Fri, 20 Jan 2012 16:59:24 +0100
> Eric Dumazet<eric.dumazet@xxxxxxxxx>  wrote:
>
> > On a typical 16 cpus machine, "cat /proc/stat" gives more than 4096
> > bytes, and is slow :
> >
> > # strace -T -o /tmp/STRACE cat /proc/stat | wc -c
> > 5826
> > # grep "cpu " /tmp/STRACE
> > read(0, "cpu  1949310 19 2144714 12117253"..., 32768) = 5826<0.001504>
> >
> >
> > Thats partly because show_stat() must be called twice since initial
> > buffer size is too small (4096 bytes for less than 32 possible cpus)
> >
> > Fix this by :
> >
> > 1) Taking into account nr_irqs in the initial buffer sizing.
> >
> > 2) Using ksize() to allow better filling of initial buffer.
> >
> > 3) Reduce the bloat on "intr ..." line :
> >     Dont output trailing " 0" values at the end of irq range.
> >
> > An alternative to 1) would be to remember the largest m->count reached
> > in show_stat()
>
> nice catch. But how about using usual seq_file rather than single_open() ?
> I just don't like multi-page buffer for this small file...very much.
>
> A rough patch here, maybe optimization will not be enough. (IOW, this may be slow.)

I myself don't like it very much, at least at first sight.
Even with optimizations applied, I doubt we can make this approach 
faster than what we currently do for /proc/stat.

Also, the code gets a lot harder to read and grasp. Problem is, unlike 
most of the stuff using seq_file, /proc/stat shows a lot of different 
kinds of information, not a single kind of easily indexable information.


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