md/raid5-cache: bump flush stripe batch size

Bump the flush stripe batch size to 2048. For my 12 disks raid
array, the stripes takes:
12 * 4k * 2048 = 96MB

This is still quite small. A hardware raid card generally has 1GB size,
which we suggest the raid5-cache has similar cache size.

The advantage of a big batch size is we can dispatch a lot of IO in the
same time, then we can do some scheduling to make better IO pattern.

Last patch prioritizes stripes, so we don't worry about a big flush
stripe batch will starve normal stripes.

Signed-off-by: Shaohua Li <shli@fb.com>

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 3f307be01b10cc70eb7b08bc31b9a2a3717372b8..738e180d3740a7766ef12fe8c9c8fbc7d0f72c00 100644 (file)
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -43,7 +43,7 @@
 /* wake up reclaim thread periodically */
 #define R5C_RECLAIM_WAKEUP_INTERVAL (30 * HZ)
 /* start flush with these full stripes */
-#define R5C_FULL_STRIPE_FLUSH_BATCH 256
+#define R5C_FULL_STRIPE_FLUSH_BATCH(conf) (conf->max_nr_stripes / 4)
 /* reclaim stripes in groups */
 #define R5C_RECLAIM_STRIPE_GROUP (NR_STRIPE_HASH_LOCKS * 2)
 
@@ -381,7 +381,7 @@ void r5c_check_cached_full_stripe(struct r5conf *conf)
         * or a full stripe (chunk size / 4k stripes).
         */
        if (atomic_read(&conf->r5c_cached_full_stripes) >=
-           min(R5C_FULL_STRIPE_FLUSH_BATCH,
+           min(R5C_FULL_STRIPE_FLUSH_BATCH(conf),
                conf->chunk_sectors >> STRIPE_SHIFT))
                r5l_wake_reclaim(conf->log, 0);
 }
@@ -1393,7 +1393,7 @@ static void r5c_do_reclaim(struct r5conf *conf)
                stripes_to_flush = R5C_RECLAIM_STRIPE_GROUP;
        else if (total_cached > conf->min_nr_stripes * 1 / 2 ||
                 atomic_read(&conf->r5c_cached_full_stripes) - flushing_full >
-                R5C_FULL_STRIPE_FLUSH_BATCH)
+                R5C_FULL_STRIPE_FLUSH_BATCH(conf))
                /*
                 * if stripe cache pressure moderate, or if there is many full
                 * stripes,flush all full stripes