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@xxxxxxxxxxxxxx>
---
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
