[tor-dev] Preferred compression type?

Steve Snyder swsnyder at snydernet.net
Fri Sep 13 20:30:49 UTC 2019

Thanks, Nick!

On 9/13/19 4:24 PM, Nick Mathewson wrote:
> On Fri, Sep 13, 2019 at 2:05 PM Steve Snyder <swsnyder at snydernet.net> wrote:
>> Given the multiple compression types supported (none, lzma, zlib, zstd),
>> what is the order of preference for runtime use?
>> Put another way, which compression method(s) should be supported to get
>> optimal runtime performance from a Tor node?
> For big objects like consensuses or consensus diffs that are sent over
> and over, relays prefer to use whichever compression method has the
> highest compression -- that's lzma2, then zstd, then zlib, then none.
> Lzma2 (aka xz) is more expensive to calculate, but the relays only
> need to calculate it once per compressed object, and then they can
> send it over and over.
> For smaller objects that are compressed in a stream (descriptors and
> microdescriptors), relays will not use xz, since it would be to
> expensive to recompute it for every stream. They'll prefer zstd, then
> zlib, then none.
> So if you want to save bandwidth above all, you should enable all
> compression algorithms.
> If you want to save CPU above all, you should enable all compression
> algorithms except xz.
> If you want to save bandwidth and CPU, I _think_ that enabling all the
> compression algorithms will result in Tor making good choices (as
> described above).  But I'd appreciate benchmarks if anybody has tried
> it both ways to find out.
> cheers,

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