Btrfs: heuristic: add bucket and sample counters and other defines

Add basic defines and structures for data sampling.

Added macros:
 - For future sampling algo
 - For bucket size

Heuristic workspace:
 - Add bucket for storing byte type counters
 - Add sample array for storing partial copy of input data range
 - Add counter for store current sample size to workspace

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor coding style fixes, comments updated ]
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index b155542c5e6b4e211f617c2e78a02936123ce702..4f5d958c71ad8fdeb7d4871f71c67da822896493 100644 (file)
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -707,8 +707,47 @@ out:
        return ret;
 }
 
+/*
+ * Heuristic uses systematic sampling to collect data from the input data
+ * range, the logic can be tuned by the following constants:
+ *
+ * @SAMPLING_READ_SIZE - how many bytes will be copied from for each sample
+ * @SAMPLING_INTERVAL  - range from which the sampled data can be collected
+ */
+#define SAMPLING_READ_SIZE     (16)
+#define SAMPLING_INTERVAL      (256)
+
+/*
+ * For statistical analysis of the input data we consider bytes that form a
+ * Galois Field of 256 objects. Each object has an attribute count, ie. how
+ * many times the object appeared in the sample.
+ */
+#define BUCKET_SIZE            (256)
+
+/*
+ * The size of the sample is based on a statistical sampling rule of thumb.
+ * The common way is to perform sampling tests as long as the number of
+ * elements in each cell is at least 5.
+ *
+ * Instead of 5, we choose 32 to obtain more accurate results.
+ * If the data contain the maximum number of symbols, which is 256, we obtain a
+ * sample size bound by 8192.
+ *
+ * For a sample of at most 8KB of data per data range: 16 consecutive bytes
+ * from up to 512 locations.
+ */
+#define MAX_SAMPLE_SIZE                (BTRFS_MAX_UNCOMPRESSED *               \
+                                SAMPLING_READ_SIZE / SAMPLING_INTERVAL)
+
+struct bucket_item {
+       u32 count;
+};
 
 struct heuristic_ws {
+       /* Partial copy of input data */
+       u8 *sample;
+       /* Buckets store counters for each byte value */
+       struct bucket_item *bucket;
        struct list_head list;
 };
 
@@ -718,6 +757,8 @@ static void free_heuristic_ws(struct list_head *ws)
 
        workspace = list_entry(ws, struct heuristic_ws, list);
 
+       kvfree(workspace->sample);
+       kfree(workspace->bucket);
        kfree(workspace);
 }
 
@@ -729,9 +770,19 @@ static struct list_head *alloc_heuristic_ws(void)
        if (!ws)
                return ERR_PTR(-ENOMEM);
 
-       INIT_LIST_HEAD(&ws->list);
+       ws->sample = kvmalloc(MAX_SAMPLE_SIZE, GFP_KERNEL);
+       if (!ws->sample)
+               goto fail;
+
+       ws->bucket = kcalloc(BUCKET_SIZE, sizeof(*ws->bucket), GFP_KERNEL);
+       if (!ws->bucket)
+               goto fail;
 
+       INIT_LIST_HEAD(&ws->list);
        return &ws->list;
+fail:
+       free_heuristic_ws(&ws->list);
+       return ERR_PTR(-ENOMEM);
 }
 
 struct workspaces_list {