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block: keep I/O throttling slice time constant

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It is not necessary to adjust the slice time at runtime.  We already
extend the current slice in order to carry over accounting into the next
slice.  Changing the actual slice time value introduces oscillations.

The guest may experience large changes in throughput or IOPS from one
moment to the next when slice times are adjusted.

Reported-by: Benoît Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-By: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2013-04-05 11:32:20 +02:00 • committed by Kevin Wolf
parent 5905fbc9c9
commit ae29d6c64b
3 changed files with 9 additions and 12 deletions
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19
block.c
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@ -140,7 +140,6 @@ void bdrv_io_limits_disable(BlockDriverState *bs)
bs->slice_start = 0;
bs->slice_end = 0;
bs->slice_time = 0;
}
static void bdrv_block_timer(void *opaque)
@ -1432,7 +1431,6 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
bs_dest->enable_write_cache = bs_src->enable_write_cache;
/* i/o timing parameters */
bs_dest->slice_time = bs_src->slice_time;
bs_dest->slice_start = bs_src->slice_start;
bs_dest->slice_end = bs_src->slice_end;
bs_dest->slice_submitted = bs_src->slice_submitted;
@ -3749,6 +3747,7 @@ static bool bdrv_exceed_bps_limits(BlockDriverState *bs, int nb_sectors,
bool is_write, double elapsed_time, uint64_t *wait)
{
uint64_t bps_limit = 0;
uint64_t extension;
double bytes_limit, bytes_base, bytes_res;
double slice_time, wait_time;
@ -3796,8 +3795,10 @@ static bool bdrv_exceed_bps_limits(BlockDriverState *bs, int nb_sectors,
* info can be kept until the timer fire, so it is increased and tuned
* based on the result of experiment.
*/
bs->slice_time = wait_time * BLOCK_IO_SLICE_TIME * 10;
bs->slice_end += bs->slice_time - 3 * BLOCK_IO_SLICE_TIME;
extension = wait_time * NANOSECONDS_PER_SECOND;
extension = DIV_ROUND_UP(extension, BLOCK_IO_SLICE_TIME) *
BLOCK_IO_SLICE_TIME;
bs->slice_end += extension;
if (wait) {
*wait = wait_time * BLOCK_IO_SLICE_TIME * 10;
}
@ -3848,8 +3849,8 @@ static bool bdrv_exceed_iops_limits(BlockDriverState *bs, bool is_write,
wait_time = 0;
}
bs->slice_time = wait_time * BLOCK_IO_SLICE_TIME * 10;
bs->slice_end += bs->slice_time - 3 * BLOCK_IO_SLICE_TIME;
/* Exceeded current slice, extend it by another slice time */
bs->slice_end += BLOCK_IO_SLICE_TIME;
if (wait) {
*wait = wait_time * BLOCK_IO_SLICE_TIME * 10;
}
@ -3868,12 +3869,10 @@ static bool bdrv_exceed_io_limits(BlockDriverState *bs, int nb_sectors,
now = qemu_get_clock_ns(vm_clock);
if ((bs->slice_start < now)
&& (bs->slice_end > now)) {
bs->slice_end = now + bs->slice_time;
bs->slice_end = now + BLOCK_IO_SLICE_TIME;
} else {
bs->slice_time = 5 * BLOCK_IO_SLICE_TIME;
bs->slice_start = now;
bs->slice_end = now + bs->slice_time;
bs->slice_end = now + BLOCK_IO_SLICE_TIME;
memset(&bs->slice_submitted, 0, sizeof(bs->slice_submitted));
}