From 858177d38d4681dad6efc015b99e4c786a34aca5 Mon Sep 17 00:00:00 2001 From: Timofey Titovets Date: Thu, 28 Sep 2017 17:33:41 +0300 Subject: [PATCH] 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 Reviewed-by: David Sterba [ update comments ] Signed-off-by: David Sterba --- fs/btrfs/compression.c | 65 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) 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 #include #include +#include #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; -- 2.30.2