Re: [Qemu-block] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache
On 06.05.2015 15:39, Alberto Garcia wrote:
The current algorithm to evict entries from the cache gives always
preference to those in the lowest positions. As the size of the cache
increases, the chances of the later elements of being removed decrease
exponentially.
In a scenario with random I/O and lots of cache misses, entries in
positions 8 and higher are rarely (if ever) evicted. This can be seen
even with the default cache size, but with larger caches the problem
becomes more obvious.
Using an LRU algorithm makes the chances of being removed from the
cache independent from the position.
Signed-off-by: Alberto Garcia be...@igalia.com
---
block/qcow2-cache.c | 31 +++
1 file changed, 15 insertions(+), 16 deletions(-)
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 6e78c8f..786c10a 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -28,10 +28,10 @@
#include trace.h
typedef struct Qcow2CachedTable {
-int64_t offset;
-booldirty;
-int cache_hits;
-int ref;
+int64_t offset;
+bool dirty;
+uint64_t lru_counter;
+int ref;
} Qcow2CachedTable;
struct Qcow2Cache {
@@ -40,6 +40,7 @@ struct Qcow2Cache {
int size;
booldepends_on_flush;
void *table_array;
+uint64_tlru_counter;
};
static inline void *qcow2_cache_get_table_addr(BlockDriverState *bs,
@@ -233,16 +234,18 @@ int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c)
for (i = 0; i c-size; i++) {
assert(c-entries[i].ref == 0);
c-entries[i].offset = 0;
-c-entries[i].cache_hits = 0;
+c-entries[i].lru_counter = 0;
}
+c-lru_counter = 0;
+
return 0;
}
static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
{
int i;
-int min_count = INT_MAX;
+uint64_t min_lru_counter = UINT64_MAX;
int min_index = -1;
@@ -251,15 +254,9 @@ static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
continue;
}
-if (c-entries[i].cache_hits min_count) {
+if (c-entries[i].lru_counter min_lru_counter) {
min_index = i;
-min_count = c-entries[i].cache_hits;
-}
-
-/* Give newer hits priority */
-/* TODO Check how to optimize the replacement strategy */
-if (c-entries[i].cache_hits 1) {
-c-entries[i].cache_hits /= 2;
+min_lru_counter = c-entries[i].lru_counter;
}
}
@@ -318,12 +315,10 @@ static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
/* Give the table some hits for the start so that it won't be replaced
* immediately. The number 32 is completely arbitrary. */
-c-entries[i].cache_hits = 32;
c-entries[i].offset = offset;
With the coment removed:
Reviewed-by: Max Reitz mre...@redhat.com
/* And return the right table */
found:
-c-entries[i].cache_hits++;
c-entries[i].ref++;
*table = qcow2_cache_get_table_addr(bs, c, i);
@@ -356,6 +351,10 @@ int qcow2_cache_put(BlockDriverState *bs, Qcow2Cache *c, void **table)
c-entries[i].ref--;
*table = NULL;
+if (c-entries[i].ref == 0) {
+c-entries[i].lru_counter = ++c-lru_counter;
+}
+
assert(c-entries[i].ref = 0);
return 0;
}
Re: [Qemu-block] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache
On Wed, May 06, 2015 at 04:39:27PM +0300, Alberto Garcia wrote: The current algorithm to evict entries from the cache gives always preference to those in the lowest positions. As the size of the cache increases, the chances of the later elements of being removed decrease exponentially. In a scenario with random I/O and lots of cache misses, entries in positions 8 and higher are rarely (if ever) evicted. This can be seen even with the default cache size, but with larger caches the problem becomes more obvious. Using an LRU algorithm makes the chances of being removed from the cache independent from the position. Signed-off-by: Alberto Garcia be...@igalia.com --- block/qcow2-cache.c | 31 +++ 1 file changed, 15 insertions(+), 16 deletions(-) Reviewed-by: Stefan Hajnoczi stefa...@redhat.com pgpaKPHOt8X3o.pgp Description: PGP signature
Re: [Qemu-block] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache
On Thu 07 May 2015 04:55:21 PM CEST, Eric Blake wrote: /* Give the table some hits for the start so that it won't be replaced * immediately. The number 32 is completely arbitrary. */ -c-entries[i].cache_hits = 32; c-entries[i].offset = offset; The comment is now dead. Oh! Feel free to remove it when committing the patch, otherwise I can send a new series. Berto
Re: [Qemu-block] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache
On 05/06/2015 07:39 AM, Alberto Garcia wrote: The current algorithm to evict entries from the cache gives always preference to those in the lowest positions. As the size of the cache increases, the chances of the later elements of being removed decrease exponentially. In a scenario with random I/O and lots of cache misses, entries in positions 8 and higher are rarely (if ever) evicted. This can be seen even with the default cache size, but with larger caches the problem becomes more obvious. Using an LRU algorithm makes the chances of being removed from the cache independent from the position. Signed-off-by: Alberto Garcia be...@igalia.com --- block/qcow2-cache.c | 31 +++ 1 file changed, 15 insertions(+), 16 deletions(-) @@ -318,12 +315,10 @@ static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c, /* Give the table some hits for the start so that it won't be replaced * immediately. The number 32 is completely arbitrary. */ -c-entries[i].cache_hits = 32; c-entries[i].offset = offset; The comment is now dead. -- Eric Blake eblake redhat com+1-919-301-3266 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature
