unix_io.c (find_cached_block, reuse_cache, unix_read_blk,

	unix_write_blk): Optimize routines so that we don't end up
	searching the cache twice when a block isn't in the
	cache.  If reads are larger than READ_DIRECT_SIZE, don't
	let them go through the cache.

diff --git a/lib/ext2fs/ChangeLog b/lib/ext2fs/ChangeLog
index 3e265c5..8ee700b 100644 (file)
--- a/lib/ext2fs/ChangeLog
+++ b/lib/ext2fs/ChangeLog
@@ -1,4 +1,10 @@
-2002-11-09    <tytso@snap.thunk.org>
+2002-11-09  Theodore Ts'o  <tytso@mit.edu>
+
+       * unix_io.c (find_cached_block, reuse_cache, unix_read_blk, 
+               unix_write_blk): Optimize routines so that we don't end up
+               searching the cache twice when a block isn't in the
+               cache.  If reads are larger than READ_DIRECT_SIZE, don't
+               let them go through the cache.
 
        * unix_io.c (find_cached_block): Fixed bug which caused some clean
                blocks to be erroneously marked as dirty, so they would

diff --git a/lib/ext2fs/unix_io.c b/lib/ext2fs/unix_io.c
index 1dcca4a..a76f9d8 100644 (file)
--- a/lib/ext2fs/unix_io.c
+++ b/lib/ext2fs/unix_io.c
@@ -55,7 +55,8 @@ struct unix_cache {
 };
 
 #define CACHE_SIZE 8
-#define WRITE_VIA_CACHE_SIZE 4 /* Must be smaller than CACHE_SIZE */
+#define WRITE_DIRECT_SIZE 4    /* Must be smaller than CACHE_SIZE */
+#define READ_DIRECT_SIZE 4     /* Should be smaller than CACHE_SIZE */
 
 struct unix_private_data {
        int     magic;
@@ -211,14 +212,14 @@ static void free_cache(io_channel channel,
 }
 
 /*
- * Try to find a block in the cache.  If get_cache is non-zero, then
- * if the block isn't in the cache, evict the oldest block in the
- * cache and create a new cache entry for the requested block.
+ * Try to find a block in the cache.  If the block is not found, and
+ * eldest is a non-zero pointer, then fill in eldest with the cache
+ * entry to that should be reused.
  */
 static struct unix_cache *find_cached_block(io_channel channel,
                                            struct unix_private_data *data,
                                            unsigned long block,
-                                           int get_cache)
+                                           struct unix_cache **eldest)
 {
        struct unix_cache       *cache, *unused_cache, *oldest_cache;
        int                     i;
@@ -226,7 +227,8 @@ static struct unix_cache *find_cached_block(io_channel channel,
        unused_cache = oldest_cache = 0;
        for (i=0, cache = data->cache; i < CACHE_SIZE; i++, cache++) {
                if (!cache->in_use) {
-                       unused_cache = cache;
+                       if (!unused_cache)
+                               unused_cache = cache;
                        continue;
                }
                if (cache->block == block) {
@@ -237,25 +239,24 @@ static struct unix_cache *find_cached_block(io_channel channel,
                    (cache->access_time < oldest_cache->access_time))
                        oldest_cache = cache;
        }
-       if (!get_cache)
-               return 0;
-       
-       /*
-        * Try to allocate cache slot.
-        */
-       if (unused_cache)
-               cache = unused_cache;
-       else {
-               cache = oldest_cache;
-               if (cache->dirty)
-                       raw_write_blk(channel, data,
-                                     cache->block, 1, cache->buf);
-       }
+       if (eldest)
+               *eldest = (unused_cache) ? unused_cache : oldest_cache;
+       return 0;
+}
+
+/*
+ * Reuse a particular cache entry for another block.
+ */
+void reuse_cache(io_channel channel, struct unix_private_data *data,
+                struct unix_cache *cache, unsigned long block)
+{
+       if (cache->dirty && cache->in_use)
+               raw_write_blk(channel, data, cache->block, 1, cache->buf);
+
        cache->in_use = 1;
        cache->dirty = 0;
        cache->block = block;
        cache->access_time = ++data->access_time;
-       return cache;
 }
 
 /*
@@ -444,7 +445,7 @@ static errcode_t unix_read_blk(io_channel channel, unsigned long block,
                               int count, void *buf)
 {
        struct unix_private_data *data;
-       struct unix_cache *cache;
+       struct unix_cache *cache, *reuse[READ_DIRECT_SIZE];
        errcode_t       retval;
        char            *cp;
        int             i, j;
@@ -454,10 +455,10 @@ static errcode_t unix_read_blk(io_channel channel, unsigned long block,
        EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_UNIX_IO_CHANNEL);
 
        /*
-        * If we're doing an odd-sized read, flush out the cache and
-        * then do a direct read.
+        * If we're doing an odd-sized read or a very large read,
+        * flush out the cache and then do a direct read.
         */
-       if (count < 0) {
+       if (count < 0 || count > WRITE_DIRECT_SIZE) {
                if ((retval = flush_cached_blocks(channel, data, 0)))
                        return retval;
                return raw_read_blk(channel, data, block, count, buf);
@@ -466,7 +467,8 @@ static errcode_t unix_read_blk(io_channel channel, unsigned long block,
        cp = buf;
        while (count > 0) {
                /* If it's in the cache, use it! */
-               if ((cache = find_cached_block(channel, data, block, 0))) {
+               if ((cache = find_cached_block(channel, data, block,
+                                              &reuse[0]))) {
 #ifdef DEBUG
                        printf("Using cached block %d\n", block);
 #endif
@@ -481,7 +483,8 @@ static errcode_t unix_read_blk(io_channel channel, unsigned long block,
                 * single read request
                 */
                for (i=1; i < count; i++)
-                       if (find_cached_block(channel, data, block+i, 0))
+                       if (find_cached_block(channel, data, block+i,
+                                             &reuse[i]))
                                break;
 #ifdef DEBUG
                printf("Reading %d blocks starting at %d\n", i, block);
@@ -492,9 +495,9 @@ static errcode_t unix_read_blk(io_channel channel, unsigned long block,
                /* Save the results in the cache */
                for (j=0; j < i; j++) {
                        count--;
-                       cache = find_cached_block(channel, data, block++, 1);
-                       if (cache)
-                               memcpy(cache->buf, cp, channel->block_size);
+                       cache = reuse[j];
+                       reuse_cache(channel, data, cache, block++);
+                       memcpy(cache->buf, cp, channel->block_size);
                        cp += channel->block_size;
                }
        }
@@ -505,7 +508,7 @@ static errcode_t unix_write_blk(io_channel channel, unsigned long block,
                                int count, const void *buf)
 {
        struct unix_private_data *data;
-       struct unix_cache *cache;
+       struct unix_cache *cache, *reuse;
        errcode_t       retval = 0, retval2;
        const char      *cp;
        int             writethrough;
@@ -518,7 +521,7 @@ static errcode_t unix_write_blk(io_channel channel, unsigned long block,
         * If we're doing an odd-sized write or a very large write,
         * flush out the cache completely and then do a direct write.
         */
-       if (count < 0 || count > WRITE_VIA_CACHE_SIZE) {
+       if (count < 0 || count > WRITE_DIRECT_SIZE) {
                if ((retval = flush_cached_blocks(channel, data, 1)))
                        return retval;
                return raw_write_blk(channel, data, block, count, buf);
@@ -535,19 +538,13 @@ static errcode_t unix_write_blk(io_channel channel, unsigned long block,
        
        cp = buf;
        while (count > 0) {
-               cache = find_cached_block(channel, data, block, 1);
+               cache = find_cached_block(channel, data, block, &reuse);
                if (!cache) {
-                       /*
-                        * Oh shit, we couldn't get cache descriptor.
-                        * Force the write directly.
-                        */
-                       if ((retval2 = raw_write_blk(channel, data, block,
-                                               1, cp)))
-                               retval = retval2;
-               } else {
-                       memcpy(cache->buf, cp, channel->block_size);
-                       cache->dirty = !writethrough;
+                       cache = reuse;
+                       reuse_cache(channel, data, cache, block);
                }
+               memcpy(cache->buf, cp, channel->block_size);
+               cache->dirty = !writethrough;
                count--;
                block++;
                cp += channel->block_size;