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Make arc_prune() asynchronous 

As described in the comment above arc_adapt_thread() it is critical
that the arc_adapt_thread() function never sleep while holding a hash
lock.  This behavior was possible in the Linux implementation because
the arc_prune() logic was implemented to be synchronous.  Under
illumos the analogous dnlc_reduce_cache() function is asynchronous.

To address this the arc_do_user_prune() function is has been reworked
in to two new functions as follows:

* arc_prune_async() is an asynchronous implementation which dispatches
the prune callback to be run by the system taskq.  This makes it
suitable to use in the context of the arc_adapt_thread().

* arc_prune() is a synchronous implementation which depends on the
arc_prune_async() implementation but blocks until the outstanding
callbacks complete.  This is used in arc_kmem_reap_now() where it
is safe, and expected, that memory will be freed.

This patch additionally adds the zfs_arc_meta_strategy module option
while allows the meta reclaim strategy to be configured.  It defaults
to a balanced strategy which has been proved to work well under Linux
but the illumos meta-only strategy can be enabled.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
This commit is contained in:
Brian Behlendorf 2015-05-30 09:57:53 -05:00
parent c5528b9ba6
commit f604673836
2 changed files with 120 additions and 41 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/sys/arc.h
View File

@ -59,10 +59,16 @@ arc_done_func_t arc_getbuf_func;
struct arc_prune {
arc_prune_func_t *p_pfunc;
void *p_private;
uint64_t p_adjust;
list_node_t p_node;
refcount_t p_refcnt;
};
typedef enum arc_strategy {
ARC_STRATEGY_META_ONLY = 0, /* Evict only meta data buffers */
ARC_STRATEGY_META_BALANCED = 1, /* Evict data buffers if needed */
} arc_strategy_t;
typedef enum arc_flags
{
/*

151
module/zfs/arc.c
View File

@ -167,6 +167,9 @@ static boolean_t arc_user_evicts_thread_exit;
/* number of objects to prune from caches when arc_meta_limit is reached */
int zfs_arc_meta_prune = 10000;
/* The preferred strategy to employ when arc_meta_limit is reached */
int zfs_arc_meta_strategy = ARC_STRATEGY_META_BALANCED;
typedef enum arc_reclaim_strategy {
ARC_RECLAIM_AGGR, /* Aggressive reclaim strategy */
ARC_RECLAIM_CONS /* Conservative reclaim strategy */
@ -531,6 +534,7 @@ static arc_state_t *arc_l2c_only;
static list_t arc_prune_list;
static kmutex_t arc_prune_mtx;
static taskq_t *arc_prune_taskq;
static arc_buf_t *arc_eviction_list;
static arc_buf_hdr_t arc_eviction_hdr;
@ -2430,47 +2434,64 @@ arc_flush_state(arc_state_t *state, uint64_t spa, arc_buf_contents_t type,
}
/*
* Request that arc user drop references so that N bytes can be released
* from the cache. This provides a mechanism to ensure the arc can honor
* the arc_meta_limit and reclaim buffers which are pinned in the cache
* by higher layers. (i.e. the zpl)
* Helper function for arc_prune() it is responsible for safely handling
* the execution of a registered arc_prune_func_t.
*/
static void
arc_do_user_prune(int64_t adjustment)
arc_prune_task(void *ptr)
{
arc_prune_func_t *func;
void *private;
arc_prune_t *cp, *np;
mutex_enter(&arc_prune_mtx);
cp = list_head(&arc_prune_list);
while (cp != NULL) {
func = cp->p_pfunc;
private = cp->p_private;
np = list_next(&arc_prune_list, cp);
refcount_add(&cp->p_refcnt, func);
mutex_exit(&arc_prune_mtx);
arc_prune_t *ap = (arc_prune_t *)ptr;
arc_prune_func_t *func = ap->p_pfunc;
if (func != NULL)
func(adjustment, private);
func(ap->p_adjust, ap->p_private);
/* Callback unregistered concurrently with execution */
if (refcount_remove(&ap->p_refcnt, func) == 0) {
ASSERT(!list_link_active(&ap->p_node));
refcount_destroy(&ap->p_refcnt);
kmem_free(ap, sizeof (*ap));
}
}
/*
* Notify registered consumers they must drop holds on a portion of the ARC
* buffered they reference. This provides a mechanism to ensure the ARC can
* honor the arc_meta_limit and reclaim otherwise pinned ARC buffers. This
* is analogous to dnlc_reduce_cache() but more generic.
*
* This operation is performed asyncronously so it may be safely called
* in the context of the arc_adapt_thread(). A reference is taken here
* for each registered arc_prune_t and the arc_prune_task() is responsible
* for releasing it once the registered arc_prune_func_t has completed.
*/
static void
arc_prune_async(int64_t adjust)
{
arc_prune_t *ap;
mutex_enter(&arc_prune_mtx);
for (ap = list_head(&arc_prune_list); ap != NULL;
ap = list_next(&arc_prune_list, ap)) {
/* User removed prune callback concurrently with execution */
if (refcount_remove(&cp->p_refcnt, func) == 0) {
ASSERT(!list_link_active(&cp->p_node));
refcount_destroy(&cp->p_refcnt);
kmem_free(cp, sizeof (*cp));
}
cp = np;
}
if (refcount_count(&ap->p_refcnt) >= 2)
continue;
refcount_add(&ap->p_refcnt, ap->p_pfunc);
ap->p_adjust = adjust;
taskq_dispatch(arc_prune_taskq, arc_prune_task, ap, TQ_SLEEP);
ARCSTAT_BUMP(arcstat_prune);
}
mutex_exit(&arc_prune_mtx);
}
static void
arc_prune(int64_t adjust)
{
arc_prune_async(adjust);
taskq_wait_outstanding(arc_prune_taskq, 0);
}
/*
* Evict the specified number of bytes from the state specified,
* restricting eviction to the spa and type given. This function
@ -2511,7 +2532,7 @@ arc_adjust_impl(arc_state_t *state, uint64_t spa, int64_t bytes,
* available for reclaim.
*/
static uint64_t
arc_adjust_meta(void)
arc_adjust_meta_balanced(void)
{
int64_t adjustmnt, delta, prune = 0;
uint64_t total_evicted = 0;
@ -2580,7 +2601,7 @@ restart:
if (zfs_arc_meta_prune) {
prune += zfs_arc_meta_prune;
arc_do_user_prune(prune);
arc_prune_async(prune);
}
}
@ -2592,6 +2613,50 @@ restart:
return (total_evicted);
}
/*
* Evict metadata buffers from the cache, such that arc_meta_used is
* capped by the arc_meta_limit tunable.
*/
static uint64_t
arc_adjust_meta_only(void)
{
uint64_t total_evicted = 0;
int64_t target;
/*
* If we're over the meta limit, we want to evict enough
* metadata to get back under the meta limit. We don't want to
* evict so much that we drop the MRU below arc_p, though. If
* we're over the meta limit more than we're over arc_p, we
* evict some from the MRU here, and some from the MFU below.
*/
target = MIN((int64_t)(arc_meta_used - arc_meta_limit),
(int64_t)(arc_anon->arcs_size + arc_mru->arcs_size - arc_p));
total_evicted += arc_adjust_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
/*
* Similar to the above, we want to evict enough bytes to get us
* below the meta limit, but not so much as to drop us below the
* space alloted to the MFU (which is defined as arc_c - arc_p).
*/
target = MIN((int64_t)(arc_meta_used - arc_meta_limit),
(int64_t)(arc_mfu->arcs_size - (arc_c - arc_p)));
total_evicted += arc_adjust_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
return (total_evicted);
}
static uint64_t
arc_adjust_meta(void)
{
if (zfs_arc_meta_strategy == ARC_STRATEGY_META_ONLY)
return (arc_adjust_meta_only());
else
return (arc_adjust_meta_balanced());
}
/*
* Return the type of the oldest buffer in the given arc state
*
@ -2905,6 +2970,14 @@ arc_kmem_reap_now(arc_reclaim_strategy_t strat, uint64_t bytes)
extern kmem_cache_t *zio_buf_cache[];
extern kmem_cache_t *zio_data_buf_cache[];
if ((arc_meta_used >= arc_meta_limit) && zfs_arc_meta_prune) {
/*
* We are exceeding our meta-data cache limit.
* Prune some entries to release holds on meta-data.
*/
arc_prune(zfs_arc_meta_prune);
}
/*
* An aggressive reclamation will shrink the cache size as well as
* reap free buffers from the arc kmem caches.
@ -2929,15 +3002,6 @@ arc_kmem_reap_now(arc_reclaim_strategy_t strat, uint64_t bytes)
}
/*
* Unlike other ZFS implementations this thread is only responsible for
* adapting the target ARC size on Linux. The responsibility for memory
* reclamation has been entirely delegated to the arc_shrinker_func()
* which is registered with the VM. To reflect this change in behavior
* the arc_reclaim thread has been renamed to arc_adapt.
*
* The following comment from arc_reclaim_thread() in illumos is still
* applicable:
*
* Threads can block in arc_get_data_buf() waiting for this thread to evict
* enough data and signal them to proceed. When this happens, the threads in
* arc_get_data_buf() are sleeping while holding the hash lock for their
@ -4862,6 +4926,9 @@ arc_init(void)
mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);
bzero(&arc_eviction_hdr, sizeof (arc_buf_hdr_t));
arc_prune_taskq = taskq_create("arc_prune", max_ncpus, minclsyspri,
max_ncpus, INT_MAX, TASKQ_PREPOPULATE);
arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
@ -4943,6 +5010,9 @@ arc_fini(void)
arc_ksp = NULL;
}
taskq_wait(arc_prune_taskq);
taskq_destroy(arc_prune_taskq);
mutex_enter(&arc_prune_mtx);
while ((p = list_head(&arc_prune_list)) != NULL) {
list_remove(&arc_prune_list, p);
@ -6374,6 +6444,9 @@ module_param(zfs_arc_meta_adjust_restarts, ulong, 0644);
MODULE_PARM_DESC(zfs_arc_meta_adjust_restarts,
"Limit number of restarts in arc_adjust_meta");
module_param(zfs_arc_meta_strategy, int, 0644);
MODULE_PARM_DESC(zfs_arc_meta_strategy, "Meta reclaim strategy");
module_param(zfs_arc_grow_retry, int, 0644);
MODULE_PARM_DESC(zfs_arc_grow_retry, "Seconds before growing arc size");