Re: [arjanv@redhat.com: Re: [PATCH] shrink per_cpu_pages to fit32byte cacheline]

[Giuliano Pochini]

On Thu, 23 Sep 2004, Marcelo Tosatti wrote:

> Forgot to CC linux-kernel, just in case someone else
> can have useful information on this matter.
>
> Andi says any additional overhead will be in the noise
> compared to cacheline saving benefit.
>
> ***********
>
> Within the Linux kernel we can benefit from changing some fields
> of commonly accessed data structures to 16 bit instead of 32 bits,
> given that the values for these fields never reach 2 ^ 16.
>
> Arjan warned me, however, that the prefix (in this case "data16") will
> cause an additional extra cycle in instruction decoding, per message above.
>
> Can you confirm that please? We can't seem to be able to find
> it in Intel's documentation.
>
> By shrinking two fields of "per_cpu_pages" structure we can fit it
> in one 32-byte cacheline (<= Pentium III and probably several other
> embedded/whatnot architectures will benefit from such a change).

One cycle is a small overhead compared to the cost of a fetch from L2
cache or, even worse, a cache miss. Memory is terribly slow. I think that
nowadays we should design things trying to keep memory accesses as few as
possible.


--
Giuliano.
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