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authorShaohua Li <shli@kernel.org>2013-09-11 14:20:28 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-09-11 15:57:15 -0700
commit2a8f9449343260373398d59228a62a4332ea513a (patch)
tree76c6ddf2a99d9dc7519585ba65c9883005908286 /include
parent15ca220e1a63af06e000691e4ae1beaba5430c32 (diff)
downloadlinux-2a8f9449343260373398d59228a62a4332ea513a.tar.bz2
swap: change block allocation algorithm for SSD
I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% CPU time (when cluster is hard to find, the CPU time can be up to 80%), which becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page cluster, which is very slow. Here I introduced a simple algorithm to search cluster. Since we only care about 256 pages cluster, we can just use a counter to track if a cluster is free. Every 256 pages use one int to store the counter. If the counter of a cluster is 0, the cluster is free. All free clusters will be added to a list, so searching cluster is very efficient. With this, scap_swap_map() overhead disappears. This might help low end SD card swap too. Because if the cluster is aligned, SD firmware can do flash erase more efficiently. We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has downside with the algorithm which might introduce regression (see below). The patch slightly changes which cluster is choosen. It always adds free cluster to list tail. This can help wear leveling for low end SSD too. And if no cluster found, the scan_swap_map() will do search from the end of last cluster. So if no cluster found, the scan_swap_map() will do search from the end of last free cluster, which is random. For SSD, this isn't a problem at all. Another downside is the cluster must be aligned to 256 pages, which will reduce the chance to find a cluster. I would expect this isn't a big problem for SSD because of the non-seek penality. (And this is the reason I only enable the algorithm for SSD). Signed-off-by: Shaohua Li <shli@fusionio.com> Cc: Rik van Riel <riel@redhat.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Kyungmin Park <kmpark@infradead.org> Cc: Hugh Dickins <hughd@google.com> Cc: Rafael Aquini <aquini@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/swap.h20
1 files changed, 20 insertions, 0 deletions
diff --git a/include/linux/swap.h b/include/linux/swap.h
index d95cde5e257d..cb5baebf31d6 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -182,6 +182,23 @@ enum {
#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */
/*
+ * We use this to track usage of a cluster. A cluster is a block of swap disk
+ * space with SWAPFILE_CLUSTER pages long and naturally aligns in disk. All
+ * free clusters are organized into a list. We fetch an entry from the list to
+ * get a free cluster.
+ *
+ * The data field stores next cluster if the cluster is free or cluster usage
+ * counter otherwise. The flags field determines if a cluster is free. This is
+ * protected by swap_info_struct.lock.
+ */
+struct swap_cluster_info {
+ unsigned int data:24;
+ unsigned int flags:8;
+};
+#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
+#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
+
+/*
* The in-memory structure used to track swap areas.
*/
struct swap_info_struct {
@@ -191,6 +208,9 @@ struct swap_info_struct {
signed char next; /* next type on the swap list */
unsigned int max; /* extent of the swap_map */
unsigned char *swap_map; /* vmalloc'ed array of usage counts */
+ struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
+ struct swap_cluster_info free_cluster_head; /* free cluster list head */
+ struct swap_cluster_info free_cluster_tail; /* free cluster list tail */
unsigned int lowest_bit; /* index of first free in swap_map */
unsigned int highest_bit; /* index of last free in swap_map */
unsigned int pages; /* total of usable pages of swap */