Commit f6a00aa1 authored by Dmitry Konishchev's avatar Dmitry Konishchev Committed by Kevin Wolf
Browse files

qemu_img: is_not_zero() optimization 



I run qemu-img under profiler and realized, that most of CPU time is
consumed by is_not_zero() function. I had made a couple of optimizations
on it and got the following output for `time qemu-img convert -O qcow2
volume.qcow2 snapshot.qcow2`:

Original qemu-img:
real 0m56.159s
user 0m34.670s
sys  0m12.079s

Patched qemu-img:
real 0m34.805s
user 0m18.445s
sys  0m12.552s

Signed-off-by: default avatarDmitry Konishchev <konishchev@gmail.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 6f321e93

qemu-img.c

+26 −3
Original line number Diff line number Diff line
@@ -496,14 +496,37 @@ static int img_commit(int argc, char **argv) 
    return 0;

}



/*

 * Checks whether the sector is not a zero sector.

 *

 * Attention! The len must be a multiple of 4 * sizeof(long) due to

 * restriction of optimizations in this function.

 */

static int is_not_zero(const uint8_t *sector, int len)

{

    /*

     * Use long as the biggest available internal data type that fits into the

     * CPU register and unroll the loop to smooth out the effect of memory

     * latency.

     */



    int i;

    len >>= 2;

    for(i = 0;i < len; i++) {

        if (((uint32_t *)sector)[i] != 0)

    long d0, d1, d2, d3;

    const long * const data = (const long *) sector;



    len /= sizeof(long);



    for(i = 0; i < len; i += 4) {

        d0 = data[i + 0];

        d1 = data[i + 1];

        d2 = data[i + 2];

        d3 = data[i + 3];



        if (d0 || d1 || d2 || d3) {

            return 1;

        }

    }



    return 0;

}