Today btrfs use simple logic to make decision compress data or not: Selected compression algorithm try compress data and if this save some space store that extent as compressed.
It's Reliable way to detect uncompressible data but it's will waste/burn cpu time for bad/un-compressible data and add latency. This way also add additional pressure on memory subsystem as for every compressed write btrfs need to allocate some buffered pages and reuse compression workspace. This is quite efficient, but not free. So, try create basic heuristic framework, this heuristic code will analizy data on the fly before call of compression code, can detect uncompressible data and advice to skip it. I leave comments with description in code, but i also will try describe that logic short. Heuristic have several internal layers: 1. Get sample data - this is cpu expensive to analize whole stream, so let's get some big enough sample from input data Scaling: In data: 128K 64K 32K 4K Sample: 4096b 3072b 2048b 1024b 2. For performance reason and for reuse it in 7th level copy selected data to sample buffer 3. Count every byte type in sample buffer 4. Count how many types of bytes we find If it's not many - data will be easy compressible 5. Count character core set size, i.e. which characters use 90% of input stream If core set small (1-50 different types) Data easy compressible If big (200-256) - data probably can't be compressed 6. If above methods are fail to make decision, try compute shannon entropy If entropy are small - data will be easy compressible If not - go to 7th 7. Entropy can't detect repeated strings of bytes So try look at the data for detect repeated bytes Compute a difference between frequency of bytes from coreset and between frequency of pair of that bytes If sum of that defferent from zero and entropy and not big, give compression code a try If entropy are High 7.2/8 - 8/8 (> 90%), and if we find BIG enough difference between frequency of a pairs and characters Give compression code a try 7th level needed for decreasing false negative returns, where data can be compressed (like ~131072b -> ~87000b ~ 0.66), but not so easy. That code, as i see, forbidden compression like: - 131072b -> ~110000b If compression ratio are better, it's allow that. Shannon entropy use log2(a/b) function, I did a try replace that with int_log2(a)-int_log2(b), but integer realization of log2 show a lack of accuracy (+-7-10%) in our case. So i precalculate some input/output values (1/131072 - 1/1) and create log2_lshift16(); I already decrease lines of that function from 1200 -> 200 for save memory (and lose some accuracy), so with precomputed function I get +- 0.5-2% of accuracy (in compare to normal "true" float log2 shannon) Thanks. Patches based on latest mainline: v4.12-rc7 P.S. I made only stability tests at now, all works stable. About performance: In userspace realization of that algorithm, which iterate over data by 128kb block and do Mmap() of file, it show ~4GiB/s over in memory (cached) data in one stream. For i5-4200M && DDR3. So i expect to not hurt compression performance. I've also duplicate patch set to: https://github.com/Nefelim4ag/linux log2_lshift() - tested by log2_generator https://github.com/Nefelim4ag/Entropy_Calculation P.S.S. Sorry for my bad english and may be for ugly code. I do my best, thanks. Changes since v1: - Fixes of checkpatch.pl warnings/errors - Use div64_u64() instead of "/" - Make log2_lshift16() more like binary tree as suggested by: Adam Borowski <kilob...@angband.pl> Changes since v2: - Fix page read address overflow in heuristic.c - Make "bucket" dynamically allocated, for fix warnings about big stack. - Small cleanups Timofey Titovets (2): Btrfs: add precomputed log2() Btrfs: add heuristic method for make decision compress or not compress fs/btrfs/Makefile | 2 +- fs/btrfs/heuristic.c | 275 ++++++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/heuristic.h | 13 +++ fs/btrfs/inode.c | 37 ++++--- fs/btrfs/log2_lshift16.c | 278 +++++++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/log2_lshift16.h | 11 ++ 6 files changed, 601 insertions(+), 15 deletions(-) create mode 100644 fs/btrfs/heuristic.c create mode 100644 fs/btrfs/heuristic.h create mode 100644 fs/btrfs/log2_lshift16.c create mode 100644 fs/btrfs/log2_lshift16.h -- 2.13.1 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
