[RFC PATCH v1] zram: experimental patch with entropy calculation

[Alexey Romanov <avromanov@sberdevices.ru>]

This patch adds the option CONFIG_ZRAM_ENTROPY which calculates
the information entropy for each page. If the entropy is higher
than the CONFIG_ZRAM_ENTROPY_THRESHOLD parameter, then
the page is stored uncompressed.

This is some attempt to strike a balance between page processing
time and compression. This might be relevant for systems that can
lose a bit in compression and gain a lot in performance effort.

I did some experiments (writing different files directly to /dev/zram0)
and got this:

+---------+-----------+------------+--------+--------------+------------+
| entropy | orig_size | compr_size |  time  | instructions | branches   |
+---------+-----------+------------+--------+--------------+------------+
|    Y    | 407003136 | 406052541  | 0.572s | 4641658347   | 784918702  |
+---------+-----------+------------+--------+--------------+------------+
|    N    | 407003136 | 405371829  | 2.098s | 17187405320  | 2027130210 |
+---------+-----------+------------+--------+--------------+------------+

apk file (~430MB):

+---------+-----------+------------+--------+--------------+------------+
| entropy | orig_size | compr_size |  time  | instructions | branches   |
+---------+-----------+------------+--------+--------------+------------+
|    Y    | 431452160 | 335864766  | 1.632s | 13055231312  | 1747341252 |
+---------+-----------+------------+--------+--------------+------------+
|    N    | 431452160 | 289107700  | 3.196s | 24345720313  | 2846272508 |
+---------+-----------+------------+--------+--------------+------------+

folder with various files as .tar (but not compressed) (~5GB):

+---------+-----------+------------+--------+--------------+------------+
| entropy | orig_size | compr_size |  time  | instructions | branches   |
+---------+-----------+------------+--------+--------------+------------+
|    Y    | 5277380608| 4230903173 | 7.066s | 35039839769  | 5636707902 |
+---------+-----------+------------+--------+--------------+------------+
|    N    | 5277380608| 4216479361 | 11.955s| 76763123464  | 9497319788 |
+---------+-----------+------------+--------+--------------+------------+

Page processing time is noticeably accelerated, while compression loss
is not very critical.

I guess the CONFIG_ZRAM_ENTROPY_THRESHOLD setting should be
different for each of the compression algorithms. At the moment,
I have configured it only for the zstd algorithm, but in the future
I plan to configure and test it for all remaining compression algorithms.

Comments are welcome.

Signed-off-by: Alexey Romanov <avromanov@sberdevices.ru>
---
 drivers/block/zram/Kconfig    | 18 ++++++++++++++++
 drivers/block/zram/zram_drv.c | 39 +++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index 668c6bf25..d873d631a 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -77,3 +77,21 @@ config ZRAM_MEMORY_TRACKING
 	  /sys/kernel/debug/zram/zramX/block_state.
 
 	  See Documentation/admin-guide/blockdev/zram.rst for more information.
+
+config ZRAM_ENTROPY
+	bool "Use entropy optimization for zram"
+	depends on ZRAM && ZRAM_DEF_COMP_ZSTD
+	help
+	  With this feature, entropy will be calculated for each page.
+	  Pages above ZRAM_ENTROPY_THRESHOLD entropy will be
+	  stored uncompressed. Use this feature if you need a performance
+	  boost and a small loss in compression.
+
+config ZRAM_ENTROPY_THRESHOLD
+	int
+	depends on ZRAM && ZRAM_ENTROPY
+	default 100000 if ZRAM_DEF_COMP_ZSTD
+	help
+	  Pages with entropy above ZRAM_ENTROPY_THRESHOLD will be stored
+	  uncompressed. The default value was chosen as a result a lot of
+	  experiments. You can try set your own value.
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index cb253d80d..483b219a4 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1350,6 +1350,35 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
 	return ret;
 }
 
+
+#ifdef CONFIG_ZRAM_ENTROPY
+static inline u32 ilog2_w(u64 n)
+{
+	return ilog2(n * n * n * n);
+}
+
+static inline s32 shannon_entropy(const u8 *src)
+{
+	s32 entropy_sum = 0;
+	u32 sz_base, i;
+	u16 entropy_count[256] = { 0 };
+
+	for (i = 0; i < PAGE_SIZE; ++i)
+		entropy_count[src[i]]++;
+
+	sz_base = ilog2_w(PAGE_SIZE);
+	for (i = 0; i < ARRAY_SIZE(entropy_count); ++i) {
+		if (entropy_count[i] > 0) {
+			s32 p = entropy_count[i];
+
+			entropy_sum += p * (sz_base - ilog2_w((u64)p));
+		}
+	}
+
+	return entropy_sum;
+}
+#endif
+
 static int __zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
 				u32 index, struct bio *bio)
 {
@@ -1376,7 +1405,17 @@ static int __zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
 compress_again:
 	zstrm = zcomp_stream_get(zram->comp);
 	src = kmap_atomic(page);
+
+#ifdef CONFIG_ZRAM_ENTROPY
+	/* Just save this page uncompressible */
+	if (shannon_entropy((const u8 *)src) > CONFIG_ZRAM_ENTROPY_THRESHOLD)
+		comp_len = PAGE_SIZE;
+	else
+		ret = zcomp_compress(zstrm, src, &comp_len);
+#else
 	ret = zcomp_compress(zstrm, src, &comp_len);
+#endif
+
 	kunmap_atomic(src);
 
 	if (unlikely(ret)) {
-- 
2.30.2