Parameters to lower memory consumption

[bnjjj]

Hello ! I would like to know if there are some parameters available to lower memory consumption ? Like for example in zlib we can play with window_bits parameter when compressing. They also have memory level and so one. I would really like to stay on miniz_oxyde instead of using zlib. My use case is to compress a long lived stream of memory I want to lower the memory usage of the compression

[bnjjj]

Something like provided here with mem_level and window_bits

[oyvindln]

The window_bits/mem_level feature isn't implemented in miniz_oxide (and original miniz, the paramter is just provided for API compatability for zlib, it's just ignored) currently.

The compress_fast function used for the fastest compression level may result in a smaller max window size in practice compared to the normal compression function (would need to verify to be sure) but I don't think it result in any smaller buffer allocation internally for the compressor object as of now if you need to minimize that.

The memory savings of the window_bits/mem_level thing in zlib is in the level of 32kb so it's only of relevance on extremely memory constrained systems hence there why there hasn't been a lot of incentive to implement it in alternative implementations.

[bnjjj]

Thanks @oyvindln for your answer.

So I played a little bit with the codebase and my tests. I'm sorry if I did some very dumb things but I wanted to share what I found and see if we could find an easy way to implement like a new CompressionLevel::LowMemory but basically when I'm putting a lower value on LZ_CODE_BUF_SIZE, LZ_HASH_BITS here I tried with 1024 for LZ_CODE_BUF_SIZE and LZ_HASH_BITS = 10, I also updated the value of LEVEL1_HASH_SIZE_MASK = 1000 and my memory is way lower, I can still have the right compressed data. I'm interested to open a PR providing this new CompressionLevel by setting lower values on these buffer sizes. What do you think ? If it's something that you would be open to, I would like to know more about all related constants to LZ_CODE_BUF_SIZE because that's the main one I would like to lower but it has dependencies for sure.

[bnjjj]

For instance, these data look good with my current use case but I would like to know the risk of lowering the size of the dictionary, because it's like the most impacting parameter.

pub const LZ_CODE_BUF_SIZE: usize = 8 * 1024;
pub const LZ_HASH_BITS: i32 = 12;
pub(crate) const LZ_DICT_SIZE: usize = 4096;

[oyvindln]

There shouldn't be any risk on the compression side other than lower compression ratio. However, if they are made into non-const values it could have notable impacts on the compiler's ability to optimize things like removing bounds checks so if they are to be made configurable it should ideally be done using generics or similar in some way so one can keep the benefits of that.

On the decompression side you can't really rely on the data in the DEFLATE stream actually obeying the maximum window size that has been set in the header DEFLATE header unless it's data you yourself is responsible for, since it's just a flag. So, it's only really usable for decompressing data you yourself have compressed with a lower window size since otherwise deflate compressed data will have been using the max window size in probably 99.9% of cases.

[praveenperera]

I'm implementing this spec in rust and it requires the window_bits be set to 10, and no header be used, so it would be helpful to be able to set window_bits to a custom value.

[dgrr]

@praveenperera indeed, a feature like the window bits would be useful. I am implementing the permessage-deflate standard for websocket and some servers might be under certain constraints. Thus, they would require a lower dictionary window.

[oyvindln]

Aight if it actually has some real world use it makes sense to look into implementing some way of limiting the compressor window size.

One maybe simple workaround for now could be to just use rle encoding (deflate_flags::TDEFL_RLE_MATCHES) since that results in only matches with a distance of 0 though that requires using the deflate function manually and I don't think it sets the zlib header window bits to less than max currently.

[oyvindln]

It's a bit janky to use stil but I've in 02a8857 I've now made it so that if TDEFL_RLE_MATCHES is used, the window bits in the header is also accordingly set to the lowest values since RLE encoding technically would correspond to a zero size window. THat won't give optimally compressed data for a decoder that can use a window size of 10 of course but it's at least a start.

I believe also the fast mode used for level 1 only uses a window size of 4096 (i.e window bits of 12) rather than the full window for matches so I can look into also setting the header accordingly, but I'll leave that for later on as I want to get a new release out soon.