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Commit a288e92c authored by Timofey Titovets's avatar Timofey Titovets Committed by David Sterba
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Btrfs: heuristic: add byte set calculation 



Calculate byte set size for data sample:
- calculate how many unique bytes have been in the sample
- for all bytes count > 0, check if we're still in the low count range
  (~25%), such data are easily compressible, otherwise furhter analysis
  is needed

Signed-off-by: default avatarTimofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 1fe4f6fa

fs/btrfs/compression.c

+45 −0
Original line number Diff line number Diff line
@@ -1222,6 +1222,45 @@ int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start, 
	return 1;

}



/*

 * Count byte values in buckets.

 * This heuristic can detect textual data (configs, xml, json, html, etc).

 * Because in most text-like data byte set is restricted to limited number of

 * possible characters, and that restriction in most cases makes data easy to

 * compress.

 *

 * @BYTE_SET_THRESHOLD - consider all data within this byte set size:

 *	less - compressible

 *	more - need additional analysis

 */

#define BYTE_SET_THRESHOLD		(64)



static u32 byte_set_size(const struct heuristic_ws *ws)

{

	u32 i;

	u32 byte_set_size = 0;



	for (i = 0; i < BYTE_SET_THRESHOLD; i++) {

		if (ws->bucket[i].count > 0)

			byte_set_size++;

	}



	/*

	 * Continue collecting count of byte values in buckets.  If the byte

	 * set size is bigger then the threshold, it's pointless to continue,

	 * the detection technique would fail for this type of data.

	 */

	for (; i < BUCKET_SIZE; i++) {

		if (ws->bucket[i].count > 0) {

			byte_set_size++;

			if (byte_set_size > BYTE_SET_THRESHOLD)

				return byte_set_size;

		}

	}



	return byte_set_size;

}



static bool sample_repeated_patterns(struct heuristic_ws *ws)

{

	const u32 half_of_sample = ws->sample_size / 2;
@@ -1321,6 +1360,12 @@ int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end) 
		ws->bucket[byte].count++;

	}



	i = byte_set_size(ws);

	if (i < BYTE_SET_THRESHOLD) {

		ret = 2;

		goto out;

	}



out:

	__free_workspace(0, ws_list, true);

	return ret;