Enforce ARC meta-data limits

This change ensures the ARC meta-data limits are enforced.  Without
this enforcement meta-data can grow to consume all of the ARC cache
pushing out data and hurting performance.  The cache is aggressively
reclaimed but this is a soft and not a hard limit.  The cache may
exceed the set limit briefly before being brought under control.

By default 25% of the ARC capacity can be used for meta-data.  This
limit can be tuned by setting the 'zfs_arc_meta_limit' module option.
Once this limit is exceeded meta-data reclaim will occur in 3 percent
chunks, or may be tuned using 'arc_reduce_dnlc_percent'.

Closes #193

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 9ca8ad17d75d37d4a4d11f138ac9cc0b99133ebd..02d8502fd643f4f8e145a39f1bcbe673ff3963d9 100644 (file)
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -180,6 +180,7 @@ unsigned long zfs_arc_meta_limit = 0;
 int zfs_arc_grow_retry = 0;
 int zfs_arc_shrink_shift = 0;
 int zfs_arc_p_min_shift = 0;
+int zfs_arc_reduce_dnlc_percent = 0;
 
 /*
  * Note that buffers can be in one of 6 states:
@@ -2084,14 +2085,16 @@ arc_kmem_reap_now(arc_reclaim_strategy_t strat)
        kmem_cache_t            *prev_data_cache = NULL;
        extern kmem_cache_t     *zio_buf_cache[];
        extern kmem_cache_t     *zio_data_buf_cache[];
-
 #ifdef _KERNEL
-       if (arc_meta_used >= arc_meta_limit) {
+       int                     retry = 0;
+
+       while ((arc_meta_used >= arc_meta_limit) && (retry < 10)) {
                /*
                 * We are exceeding our meta-data cache limit.
                 * Purge some DNLC entries to release holds on meta-data.
                 */
                dnlc_reduce_cache((void *)(uintptr_t)arc_reduce_dnlc_percent);
+               retry++;
        }
 #if defined(__i386)
        /*
@@ -2157,6 +2160,10 @@ arc_reclaim_thread(void)
                        arc_no_grow = FALSE;
                }
 
+               /* Keep meta data usage within limits */
+               if (arc_meta_used >= arc_meta_limit)
+                       arc_kmem_reap_now(ARC_RECLAIM_CONS);
+
                arc_adjust();
 
                if (arc_eviction_list != NULL)
@@ -3583,6 +3590,9 @@ arc_init(void)
        if (zfs_arc_p_min_shift > 0)
                arc_p_min_shift = zfs_arc_p_min_shift;
 
+       if (zfs_arc_reduce_dnlc_percent > 0)
+               arc_reduce_dnlc_percent = zfs_arc_reduce_dnlc_percent;
+
        /* if kmem_flags are set, lets try to use less memory */
        if (kmem_debugging())
                arc_c = arc_c / 2;
@@ -4765,4 +4775,7 @@ MODULE_PARM_DESC(zfs_arc_max, "Maximum arc size");
 
 module_param(zfs_arc_meta_limit, ulong, 0644);
 MODULE_PARM_DESC(zfs_arc_meta_limit, "Meta limit for arc size");
+
+module_param(arc_reduce_dnlc_percent, uint, 0644);
+MODULE_PARM_DESC(arc_reduce_dnlc_percent, "Meta reclaim percentage");
 #endif