On (03/21/18 10:10), Maninder Singh wrote: > LZ4 specification defines 2 byte offset length for 64 KB data. > But in case of ZRAM we compress data per page and in most of > architecture PAGE_SIZE is 4KB. So we can decide offset length based > on actual offset value. For this we can reserve 1 bit to decide offset > length (1 byte or 2 byte). 2 byte required only if ofsset is greater than 127, > else 1 byte is enough. > > With this new implementation new offset value can be at MAX 32 KB. > > Thus we can save more memory for compressed data. > > results checked with new implementation:- > > comression size for same input source > (LZ4_DYN < LZO < LZ4) > > LZO > ======= > orig_data_size: 78917632 > compr_data_size: 15894668 > mem_used_total: 17117184 > > LZ4 > ======== > orig_data_size: 78917632 > compr_data_size: 16310717 > mem_used_total: 17592320 > > LZ4_DYN > ======= > orig_data_size: 78917632 > compr_data_size: 15520506 > mem_used_total: 16748544 This seems like a reasonable extension to the algorithm, and it looks like LZ4_DYN is about a 5% improvement to compression ratio on your benchmark. The biggest question I have is if it is worthwhile to maintain a separate incompatible variant of LZ4 in the kernel without any upstream for a 5% gain? If we do want to go forward with this, we should perform more benchmarks. I commented in the patch, but because the `dynOffset` variable isn't a compile time static in LZ4_decompress_generic(), I suspect that the patch causes a regression in decompression speed for both LZ4 and LZ4_DYN. You'll need to re-run the benchmarks to first show that LZ4 before the patch performs the same as LZ4 after the patch. Then re-run the LZ4 vs LZ4_DYN benchmarks. I would also like to see a benchmark in user-space (with the code), so we can see the performance of LZ4 before and after the patch, as well as LZ4 vs LZ4_DYN without anything else going on. I expect the extra branches in the decoding loop to have an impact on speed, and I would like to see how big the impact is without noise. CC-ing Yann Collet, the author of LZ4
