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Support re-prioritizing asynchronous prefetches

Browse Source 
When sequential scrubs were merged, all calls to arc_read()
(including prefetch IOs) were given ZIO_PRIORITY_ASYNC_READ.
Unfortunately, this behaves badly with an existing issue where
prefetch IOs cannot be re-prioritized after the issue. The
result is that synchronous reads end up in the same vdev_queue
as the scrub IOs and can have (in some workloads) multiple
seconds of latency.

This patch incorporates 2 changes. The first ensures that all
scrub IOs are given ZIO_PRIORITY_SCRUB to allow the vdev_queue
code to differentiate between these I/Os and user prefetches.
Second, this patch introduces zio_change_priority() to provide
the missing capability to upgrade a zio's priority.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #6921 
Closes #6926
This commit is contained in:
Tom Caputi 2017-12-21 12:13:06 -05:00 committed by Brian Behlendorf
parent 993669a7bf
commit a8b2e30685
11 changed files with 124 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/sys/arc_impl.h
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@ -98,6 +98,7 @@ struct arc_callback {
boolean_t acb_noauth;
uint64_t acb_dsobj;
zio_t *acb_zio_dummy;
zio_t *acb_zio_head;
arc_callback_t *acb_next;
};

2
include/sys/trace_arc.h
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@ -108,7 +108,7 @@ DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__evict);
DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__delete);
DEFINE_ARC_BUF_HDR_EVENT(zfs_new_state__mru);
DEFINE_ARC_BUF_HDR_EVENT(zfs_new_state__mfu);
DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__sync__wait__for__async);
DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__async__upgrade__sync);
DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__demand__hit__predictive__prefetch);
DEFINE_ARC_BUF_HDR_EVENT(zfs_l2arc__hit);
DEFINE_ARC_BUF_HDR_EVENT(zfs_l2arc__miss);

1
include/sys/vdev.h
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@ -123,6 +123,7 @@ extern void vdev_queue_init(vdev_t *vd);
extern void vdev_queue_fini(vdev_t *vd);
extern zio_t *vdev_queue_io(zio_t *zio);
extern void vdev_queue_io_done(zio_t *zio);
extern void vdev_queue_change_io_priority(zio_t *zio, zio_priority_t priority);
extern int vdev_queue_length(vdev_t *vd);
extern uint64_t vdev_queue_last_offset(vdev_t *vd);

2
include/sys/zio.h
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@ -586,6 +586,8 @@ extern void zio_vdev_io_bypass(zio_t *zio);
extern void zio_vdev_io_reissue(zio_t *zio);
extern void zio_vdev_io_redone(zio_t *zio);
extern void zio_change_priority(zio_t *pio, zio_priority_t priority);
extern void zio_checksum_verified(zio_t *zio);
extern int zio_worst_error(int e1, int e2);

0
io_spa- Normal file
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54
module/zfs/arc.c
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@ -663,7 +663,7 @@ typedef struct arc_stats {
kstat_named_t arcstat_dnode_limit;
kstat_named_t arcstat_meta_max;
kstat_named_t arcstat_meta_min;
kstat_named_t arcstat_sync_wait_for_async;
kstat_named_t arcstat_async_upgrade_sync;
kstat_named_t arcstat_demand_hit_predictive_prefetch;
kstat_named_t arcstat_demand_hit_prescient_prefetch;
kstat_named_t arcstat_need_free;
@ -763,7 +763,7 @@ static arc_stats_t arc_stats = {
{ "arc_dnode_limit", KSTAT_DATA_UINT64 },
{ "arc_meta_max", KSTAT_DATA_UINT64 },
{ "arc_meta_min", KSTAT_DATA_UINT64 },
{ "sync_wait_for_async", KSTAT_DATA_UINT64 },
{ "async_upgrade_sync", KSTAT_DATA_UINT64 },
{ "demand_hit_predictive_prefetch", KSTAT_DATA_UINT64 },
{ "demand_hit_prescient_prefetch", KSTAT_DATA_UINT64 },
{ "arc_need_free", KSTAT_DATA_UINT64 },
@ -5911,32 +5911,20 @@ top:
*arc_flags |= ARC_FLAG_CACHED;
if (HDR_IO_IN_PROGRESS(hdr)) {
zio_t *head_zio = hdr->b_l1hdr.b_acb->acb_zio_head;
ASSERT3P(head_zio, !=, NULL);
if ((hdr->b_flags & ARC_FLAG_PRIO_ASYNC_READ) &&
priority == ZIO_PRIORITY_SYNC_READ) {
/*
* This sync read must wait for an
* in-progress async read (e.g. a predictive
* prefetch). Async reads are queued
* separately at the vdev_queue layer, so
* this is a form of priority inversion.
* Ideally, we would "inherit" the demand
* i/o's priority by moving the i/o from
* the async queue to the synchronous queue,
* but there is currently no mechanism to do
* so. Track this so that we can evaluate
* the magnitude of this potential performance
* problem.
*
* Note that if the prefetch i/o is already
* active (has been issued to the device),
* the prefetch improved performance, because
* we issued it sooner than we would have
* without the prefetch.
* This is a sync read that needs to wait for
* an in-flight async read. Request that the
* zio have its priority upgraded.
*/
DTRACE_PROBE1(arc__sync__wait__for__async,
zio_change_priority(head_zio, priority);
DTRACE_PROBE1(arc__async__upgrade__sync,
arc_buf_hdr_t *, hdr);
ARCSTAT_BUMP(arcstat_sync_wait_for_async);
ARCSTAT_BUMP(arcstat_async_upgrade_sync);
}
if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
arc_hdr_clear_flags(hdr,
@ -5966,6 +5954,7 @@ top:
spa, NULL, NULL, NULL, zio_flags);
ASSERT3P(acb->acb_done, !=, NULL);
acb->acb_zio_head = head_zio;
acb->acb_next = hdr->b_l1hdr.b_acb;
hdr->b_l1hdr.b_acb = acb;
mutex_exit(hash_lock);
@ -6182,14 +6171,17 @@ top:
vd = NULL;
}
if (priority == ZIO_PRIORITY_ASYNC_READ)
/*
* We count both async reads and scrub IOs as asynchronous so
* that both can be upgraded in the event of a cache hit while
* the read IO is still in-flight.
*/
if (priority == ZIO_PRIORITY_ASYNC_READ ||
priority == ZIO_PRIORITY_SCRUB)
arc_hdr_set_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
else
arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
if (hash_lock != NULL)
mutex_exit(hash_lock);
/*
* At this point, we have a level 1 cache miss. Try again in
* L2ARC if possible.
@ -6260,6 +6252,10 @@ top:
ZIO_FLAG_CANFAIL |
ZIO_FLAG_DONT_PROPAGATE |
ZIO_FLAG_DONT_RETRY, B_FALSE);
acb->acb_zio_head = rzio;
if (hash_lock != NULL)
mutex_exit(hash_lock);
DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
zio_t *, rzio);
@ -6276,6 +6272,8 @@ top:
goto out;
/* l2arc read error; goto zio_read() */
if (hash_lock != NULL)
mutex_enter(hash_lock);
} else {
DTRACE_PROBE1(l2arc__miss,
arc_buf_hdr_t *, hdr);
@ -6296,6 +6294,10 @@ top:
rzio = zio_read(pio, spa, bp, hdr_abd, size,
arc_read_done, hdr, priority, zio_flags, zb);
acb->acb_zio_head = rzio;
if (hash_lock != NULL)
mutex_exit(hash_lock);
if (*arc_flags & ARC_FLAG_WAIT) {
rc = zio_wait(rzio);

8
module/zfs/dsl_scan.c
View File

@ -1529,7 +1529,7 @@ dsl_scan_prefetch_thread(void *arg)
/* issue the prefetch asynchronously */
(void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa,
&spic->spic_bp, dsl_scan_prefetch_cb, spic->spic_spc,
ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, &spic->spic_zb);
ZIO_PRIORITY_SCRUB, zio_flags, &flags, &spic->spic_zb);
kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
}
@ -1611,7 +1611,7 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
arc_buf_t *buf;
err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
if (err) {
scn->scn_phys.scn_errors++;
return (err);
@ -1639,7 +1639,7 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
}
err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
if (err) {
scn->scn_phys.scn_errors++;
return (err);
@ -1658,7 +1658,7 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
arc_buf_t *buf;
err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
if (err) {
scn->scn_phys.scn_errors++;
return (err);

42
module/zfs/vdev_queue.c
View File

@ -803,6 +803,48 @@ vdev_queue_io_done(zio_t *zio)
mutex_exit(&vq->vq_lock);
}
void
vdev_queue_change_io_priority(zio_t *zio, zio_priority_t priority)
{
vdev_queue_t *vq = &zio->io_vd->vdev_queue;
avl_tree_t *tree;
ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
ASSERT3U(priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
if (zio->io_type == ZIO_TYPE_READ) {
if (priority != ZIO_PRIORITY_SYNC_READ &&
priority != ZIO_PRIORITY_ASYNC_READ &&
priority != ZIO_PRIORITY_SCRUB)
priority = ZIO_PRIORITY_ASYNC_READ;
} else {
ASSERT(zio->io_type == ZIO_TYPE_WRITE);
if (priority != ZIO_PRIORITY_SYNC_WRITE &&
priority != ZIO_PRIORITY_ASYNC_WRITE)
priority = ZIO_PRIORITY_ASYNC_WRITE;
}
mutex_enter(&vq->vq_lock);
/*
* If the zio is in none of the queues we can simply change
* the priority. If the zio is waiting to be submitted we must
* remove it from the queue and re-insert it with the new priority.
* Otherwise, the zio is currently active and we cannot change its
* priority.
*/
tree = vdev_queue_class_tree(vq, zio->io_priority);
if (avl_find(tree, zio, NULL) == zio) {
avl_remove(vdev_queue_class_tree(vq, zio->io_priority), zio);
zio->io_priority = priority;
avl_add(vdev_queue_class_tree(vq, zio->io_priority), zio);
} else if (avl_find(&vq->vq_active_tree, zio, NULL) != zio) {
zio->io_priority = priority;
}
mutex_exit(&vq->vq_lock);
}
/*
* As these two methods are only used for load calculations we're not
* concerned if we get an incorrect value on 32bit platforms due to lack of

43
module/zfs/zio.c
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@ -539,6 +539,8 @@ zio_walk_children(zio_t *pio, zio_link_t **zl)
{
list_t *cl = &pio->io_child_list;
ASSERT(MUTEX_HELD(&pio->io_lock));
*zl = (*zl == NULL) ? list_head(cl) : list_next(cl, *zl);
if (*zl == NULL)
return (NULL);
@ -573,8 +575,8 @@ zio_add_child(zio_t *pio, zio_t *cio)
zl->zl_parent = pio;
zl->zl_child = cio;
mutex_enter(&cio->io_lock);
mutex_enter(&pio->io_lock);
mutex_enter(&cio->io_lock);
ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);
@ -587,8 +589,8 @@ zio_add_child(zio_t *pio, zio_t *cio)
pio->io_child_count++;
cio->io_parent_count++;
mutex_exit(&pio->io_lock);
mutex_exit(&cio->io_lock);
mutex_exit(&pio->io_lock);
}
static void
@ -597,8 +599,8 @@ zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
ASSERT(zl->zl_parent == pio);
ASSERT(zl->zl_child == cio);
mutex_enter(&cio->io_lock);
mutex_enter(&pio->io_lock);
mutex_enter(&cio->io_lock);
list_remove(&pio->io_child_list, zl);
list_remove(&cio->io_parent_list, zl);
@ -606,8 +608,8 @@ zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
pio->io_child_count--;
cio->io_parent_count--;
mutex_exit(&pio->io_lock);
mutex_exit(&cio->io_lock);
mutex_exit(&pio->io_lock);
kmem_cache_free(zio_link_cache, zl);
}
@ -1963,14 +1965,16 @@ zio_reexecute(zio_t *pio)
* cannot be affected by any side effects of reexecuting 'cio'.
*/
zio_link_t *zl = NULL;
mutex_enter(&pio->io_lock);
for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
cio_next = zio_walk_children(pio, &zl);
mutex_enter(&pio->io_lock);
for (int w = 0; w < ZIO_WAIT_TYPES; w++)
pio->io_children[cio->io_child_type][w]++;
mutex_exit(&pio->io_lock);
zio_reexecute(cio);
mutex_enter(&pio->io_lock);
}
mutex_exit(&pio->io_lock);
/*
* Now that all children have been reexecuted, execute the parent.
@ -3474,6 +3478,35 @@ zio_vdev_io_done(zio_t *zio)
return (ZIO_PIPELINE_CONTINUE);
}
/*
* This function is used to change the priority of an existing zio that is
* currently in-flight. This is used by the arc to upgrade priority in the
* event that a demand read is made for a block that is currently queued
* as a scrub or async read IO. Otherwise, the high priority read request
* would end up having to wait for the lower priority IO.
*/
void
zio_change_priority(zio_t *pio, zio_priority_t priority)
{
zio_t *cio, *cio_next;
zio_link_t *zl = NULL;
ASSERT3U(priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
if (pio->io_vd != NULL && pio->io_vd->vdev_ops->vdev_op_leaf) {
vdev_queue_change_io_priority(pio, priority);
} else {
pio->io_priority = priority;
}
mutex_enter(&pio->io_lock);
for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
cio_next = zio_walk_children(pio, &zl);
zio_change_priority(cio, priority);
}
mutex_exit(&pio->io_lock);
}
/*
* For non-raidz ZIOs, we can just copy aside the bad data read from the
* disk, and use that to finish the checksum ereport later.

0
spa_stats.io_history Normal file
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14
tests/zfs-tests/tests/perf/scripts/prefetch_io.sh
View File

@ -41,9 +41,9 @@ function get_prefetched_demand_reads
echo $demand_reads
}
function get_sync_wait_for_async
function get_async_upgrade_sync
{
typeset -l sync_wait=`awk '$1 == "sync_wait_for_async" \
typeset -l sync_wait=`awk '$1 == "async_upgrade_sync" \
{ print $3 }' $zfs_kstats/arcstats`
echo $sync_wait
@ -59,7 +59,7 @@ poolname=$1
interval=$2
prefetch_ios=$(get_prefetch_ios)
prefetched_demand_reads=$(get_prefetched_demand_reads)
sync_wait_for_async=$(get_sync_wait_for_async)
async_upgrade_sync=$(get_async_upgrade_sync)
while true
do
@ -73,10 +73,10 @@ do
$(( $new_prefetched_demand_reads - $prefetched_demand_reads ))
prefetched_demand_reads=$new_prefetched_demand_reads
new_sync_wait_for_async=$(get_sync_wait_for_async)
printf "%-24s\t%u\n" "sync_wait_for_async" \
$(( $new_sync_wait_for_async - $sync_wait_for_async ))
sync_wait_for_async=$new_sync_wait_for_async
new_async_upgrade_sync=$(get_async_upgrade_sync)
printf "%-24s\t%u\n" "async_upgrade_sync" \
$(( $new_async_upgrade_sync - $async_upgrade_sync ))
async_upgrade_sync=$new_async_upgrade_sync
sleep $interval
done