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    Subject[: Re: [PATCH] shrink per_cpu_pages to fit 32byte cacheline]

    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.



    We need some assistance here - you can probably help us.

    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).

    And we just shrank two fields of "struct pagevec" in a similar way
    in Andrew's -mm tree.

    I'm adding linux-kernel just in case someone else can have
    useful comments.


    ----- Forwarded message from Arjan van de Ven <> -----

    From: Arjan van de Ven <>
    Date: Tue, 14 Sep 2004 13:13:29 +0200
    To: Marcelo Tosatti <>
    Cc:, "Martin J. Bligh" <>,
    In-Reply-To: <20040914093407.GA23935@logos.cnet>
    Subject: Re: [PATCH] shrink per_cpu_pages to fit 32byte cacheline
    Original-Recipient: rfc822;
    X-MIMETrack: Itemize by SMTP Server on USMail/Cyclades(Release 6.5.1|January 21, 2004) at
    09/14/2004 03:13:02

    On Tue, Sep 14, 2004 at 06:34:07AM -0300, Marcelo Tosatti wrote:
    > How come short access can cost 1 extra cycle? Because you need two "read bytes" ?

    on an x86, a word (2byte) access will cause a prefix byte to the
    instruction, that particular prefix byte will take an extra cycle during execution
    of the instruction and potentially reduces the parallal decodability of

    ----- End forwarded message -----

    ----- End forwarded message -----
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