Re: [PATCH 0/1] cover-letter/lz4: Implement lz4 with dynamic offset length.

[Nick Terrell <terrelln@fb.com>]

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