I understand that GZIP is a combination of LZ77 and Huffman coding and can be configured with a level between 1-9 where 1 indicates the fastest compression (less compression) and 9 indicates the slowest compression method (best compression).
My question is, does the choice of level only impact the compression process or is there an additional cost also incurred in decompression depending on the level used to compress?
I ask because typically many web servers will GZIP responses on the fly if the client supports it, e.g. Accept-Encoding: gzip. I appreciate that when doing this on the fly a level such as 6 might be the good choice for the average case, since it gives a good balance between speed and compression.
However, if I have a bunch of static assets that I can GZIP just once ahead of time – and never need to do this again – would there be any downside to using the highest but slowest compression level? I.e. is there now an additional overhead for the client that would not have been incurred had a lower compression level been used.
Best Answer
Great question, and an underexposed issue. Your intuition is solid – for some compression algorithms, choosing the max level of compression can require more work from the decompressor when it's unpacked.
Luckily, that's not true for gzip – there's no extra overhead for the client/browser to decompress more heavily compressed gzip files (e.g. choosing 9 for compression instead of 6, assuming the standard zlib codebase that most servers use). The best measure for this is decompression rate, which for present purposes is in units of MB/sec, while also monitoring overhead like memory and CPU. Simply going by decompression time is no good because the file is smaller at higher compression settings, and we're not controlling for that factor if we're only using a stopwatch.
gzip decompression quickly gets asymptotic in terms of both time-to-decompress and memory usage once you get past level 6 compressed content. The time-to-decompress flatlines for levels 7, 8, and 9 in the test results linked by Marcus Müller, though that's coarse-grained data given in whole seconds.
You'll also notice in those results that the memory requirements for decompression are flat for all levels of compression at 0.1 MiB. That's almost unbelievable, just a degree of excellence in software that we rarely see. Mark Adler and colleagues deserve massive props for what they achieved. gzip is a very nice format.
The memory use gets at your question about overhead. There really is none. You don't gain much with level 9 in terms of browser decompression speed, but you don't lose anything.
Now, check out these test results for a bit more texture. You'll see how the gzip decompression rate is slightly faster with level 9 compressed content than with lower levels (at level 9, decomp rate is about 0.9% faster than at level 6, for example). That is interesting and surprising. I wouldn't expect the rate to increase. That was just one set of test results – it may not hold for other scenarios (and the difference is quite small in any case).
Parting note: Precompressing static files is a good idea, but I don't recommend gzip at level 9. You'll get smaller files than gzip-9 by instead using zopfli or libdeflate. Zopfli is a well-established gzip compressor from Google. libdeflate is new but quite excellent. In my testing it consistently beats gzip-9, but still trails zopfli. You can also use 7-Zip to create gzip files, and it will consistently beat gzip-9. (In the foregoing, gzip-9 refers to using the canonical gzip or zlib application that Apache and nginx use).