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[RFC v1] util/aio: Keep notification disabled as much as possible
From: |
Chao Gao |
Subject: |
[RFC v1] util/aio: Keep notification disabled as much as possible |
Date: |
Fri, 1 Jul 2022 17:13:48 +0800 |
When measuring FIO read performance (cache=writethrough, bs=4k, iodepth=64) in
VMs, we observe ~80K/s notifications (e.g., EPT_MISCONFIG) from guest to qemu.
Currently, poll_set_started(ctx,false) is called in try_poll_mode() to enable
virtqueue notification in below 4 cases:
1. ctx->poll_ns is 0
2. a zero timeout is passed to try_poll_mode()
3. polling succeeded but reported as no progress
4. polling failed and reported as no progress
To minimize unnecessary guest notifications, keep notification disabled when
it is possible, i.e., polling is enabled and last polling doesn't fail.
Keep notification disabled for case #2 and #3; handle case #2 simply by a call
of run_poll_handlers_once() and for case #3, differentiate successful/failed
polling and skip the call of poll_set_started(ctx,false) for successful ones.
With this patch applied, FIO seq-read performance (bs=4k, iodepth=64,
cache=writethrough) in VMs increases from 330K/s to 413K/s IOPS.
Below are changes in VM-exit statistics:
w/o this patch (duration:10s):
VM-EXIT Samples Samples% Time% Min Time Max Time
Avg time
EPT_MISCONFIG 797440 99.34% 98.58% 0.39us 57.92us
0.60us ( +- 0.05% )
MSR_WRITE 3672 0.46% 1.15% 0.88us 4.97us
1.52us ( +- 0.53% )
EXTERNAL_INTERRUPT 1638 0.20% 0.27% 0.59us 11.04us
0.81us ( +- 1.71% )
w/ this patch (duration:10s):
VM-EXIT Samples Samples% Time% Min Time Max Time
Avg time
MSR_WRITE 3524 84.11% 87.17% 0.86us 4.43us
1.74us ( +- 0.60% )
EXTERNAL_INTERRUPT 515 12.29% 10.05% 0.64us 8.95us
1.37us ( +- 1.54% )
EPT_MISCONFIG 151 3.60% 2.79% 0.40us 52.07us
1.30us ( +- 31.44% )
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
util/aio-posix.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 731f3826c0..bd2076145b 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -519,7 +519,7 @@ static bool remove_idle_poll_handlers(AioContext *ctx,
* Returns: true if progress was made, false otherwise
*/
static bool run_poll_handlers(AioContext *ctx, AioHandlerList *ready_list,
- int64_t max_ns, int64_t *timeout)
+ int64_t max_ns, int64_t *timeout, bool *success)
{
bool progress;
int64_t start_time, elapsed_time;
@@ -553,6 +553,8 @@ static bool run_poll_handlers(AioContext *ctx,
AioHandlerList *ready_list,
progress = true;
}
+ *success = !*timeout;
+
/* If time has passed with no successful polling, adjust *timeout to
* keep the same ending time.
*/
@@ -577,22 +579,28 @@ static bool run_poll_handlers(AioContext *ctx,
AioHandlerList *ready_list,
static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
int64_t *timeout)
{
- int64_t max_ns;
+ int64_t max_ns, start_time;
+ bool success = false;
if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
return false;
}
+ if (!*timeout) {
+ start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+ return run_poll_handlers_once(ctx, ready_list, start_time, timeout);
+ }
+
max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
poll_set_started(ctx, ready_list, true);
- if (run_poll_handlers(ctx, ready_list, max_ns, timeout)) {
+ if (run_poll_handlers(ctx, ready_list, max_ns, timeout, &success)) {
return true;
}
}
- if (poll_set_started(ctx, ready_list, false)) {
+ if (!success && poll_set_started(ctx, ready_list, false)) {
*timeout = 0;
return true;
}
--
2.25.1
- [RFC v1] util/aio: Keep notification disabled as much as possible,
Chao Gao <=