Btrfs: heuristic: add byte core set calculation

Calculate byte core set for data sample:
- sort buckets' numbers in decreasing order
- count how many values cover 90% of the sample

If the core set size is low (<=25%), data are easily compressible.
If the core set size is high (>=80%), data are not compressible.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 65 insertions, 0 deletions

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index e949f078a81b..c551d8a979f4 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -33,6 +33,7 @@
 #include <linux/bit_spinlock.h>
 #include <linux/slab.h>
 #include <linux/sched/mm.h>
+#include <linux/sort.h>
 #include "ctree.h"
 #include "disk-io.h"
 #include "transaction.h"
@@ -1222,6 +1223,59 @@ int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
         return 1;
 }
 
+/* Compare buckets by size, ascending */
+static int bucket_comp_rev(const void *lv, const void *rv)
+{
+        const struct bucket_item *l = (const struct bucket_item *)lv;
+        const struct bucket_item *r = (const struct bucket_item *)rv;
+
+        return r->count - l->count;
+}
+
+/*
+ * Size of the core byte set - how many bytes cover 90% of the sample
+ *
+ * There are several types of structured binary data that use nearly all byte
+ * values. The distribution can be uniform and counts in all buckets will be
+ * nearly the same (eg. encrypted data). Unlikely to be compressible.
+ *
+ * Other possibility is normal (Gaussian) distribution, where the data could
+ * be potentially compressible, but we have to take a few more steps to decide
+ * how much.
+ *
+ * @BYTE_CORE_SET_LOW  - main part of byte values repeated frequently,
+ *                       compression algo can easy fix that
+ * @BYTE_CORE_SET_HIGH - data have uniform distribution and with high
+ *                       probability is not compressible
+ */
+#define BYTE_CORE_SET_LOW               (64)
+#define BYTE_CORE_SET_HIGH              (200)
+
+static int byte_core_set_size(struct heuristic_ws *ws)
+{
+        u32 i;
+        u32 coreset_sum = 0;
+        const u32 core_set_threshold = ws->sample_size * 90 / 100;
+        struct bucket_item *bucket = ws->bucket;
+
+        /* Sort in reverse order */
+        sort(bucket, BUCKET_SIZE, sizeof(*bucket), &bucket_comp_rev, NULL);
+
+        for (i = 0; i < BYTE_CORE_SET_LOW; i++)
+                coreset_sum += bucket[i].count;
+
+        if (coreset_sum > core_set_threshold)
+                return i;
+
+        for (; i < BYTE_CORE_SET_HIGH && bucket[i].count > 0; i++) {
+                coreset_sum += bucket[i].count;
+                if (coreset_sum > core_set_threshold)
+                        break;
+        }
+
+        return i;
+}
+
 /*
  * Count byte values in buckets.
  * This heuristic can detect textual data (configs, xml, json, html, etc).
@@ -1366,6 +1420,17 @@ int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end)
                 goto out;
         }
 
+        i = byte_core_set_size(ws);
+        if (i <= BYTE_CORE_SET_LOW) {
+                ret = 3;
+                goto out;
+        }
+
+        if (i >= BYTE_CORE_SET_HIGH) {
+                ret = 0;
+                goto out;
+        }
+
 out:
         __free_workspace(0, ws_list, true);
         return ret;