Re: [Qemu-devel] [PATCH] xbzrle: don't check the value in the vm ram repeatedly

[Eric Blake <eblake@redhat.com>]

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On 03/29/2014 09:12 AM, 陈梁 wrote:

>        /* word at a time for speed */
>        if (!res) {
>            while (i < slen &&
>                   (*(long *)(old_buf + i)) == (*(long *)(new_buf + i))) {
>                i += sizeof(long);
>                zrun_len += sizeof(long);
>            }
> 
>            /* go over the rest */
>            //while (i < slen && old_buf[i] == new_buf[i]) {
>            //    zrun_len++;
>            //    i++;
>            //}
>        }
>        i += sizeof(long);
>        nzrun_len += sizeof(long);

That does not produce a minimal compression (it treats all 8 bytes as
different, even if 7 of them were the same).  It might be a viable
solution if the extra overhead of the additional bytes sent over the
wire is less than the overhead saved by not re-checking the temporary
variable to determine a better compression.  But I'm not convinced - it
seems that reading memory into a register, then doing multiple
operations on that cached value, will be a win (that is, minimal
compression matters, because time spent transmitting bytes over the
network is slower than time spent calculating how to avoid bytes to
transmit).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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