Re: [ceph-users] ceph zstd not for bluestor due to performance reasons

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On Thu, 26 Oct 2017, Stefan Priebe - Profihost AG wrote:
> Hi Sage,
> 
> Am 25.10.2017 um 21:54 schrieb Sage Weil:
> > On Wed, 25 Oct 2017, Stefan Priebe - Profihost AG wrote:
> >> Hello,
> >>
> >> in the lumious release notes is stated that zstd is not supported by
> >> bluestor due to performance reason. I'm wondering why btrfs instead
> >> states that zstd is as fast as lz4 but compresses as good as zlib.
> >>
> >> Why is zlib than supported by bluestor? And why does btrfs / facebook
> >> behave different?
> >>
> >> "BlueStore supports inline compression using zlib, snappy, or LZ4. (Ceph
> >> also supports zstd for RGW compression but zstd is not recommended for
> >> BlueStore for performance reasons.)"
> > 
> > zstd will work but in our testing the performance wasn't great for 
> > bluestore in particular.  The problem was that for each compression run 
> > there is a relatively high start-up cost initializing the zstd 
> > context/state (IIRC a memset of a huge memory buffer) that dominated the 
> > execution time... primarily because bluestore is generally compressing 
> > pretty small chunks of data at a time, not big buffers or streams.
> > 
> > Take a look at unittest_compression timings on compressing 16KB buffers 
> > (smaller than bluestore needs usually, but illustrated of the problem):
> > 
> > [ RUN      ] Compressor/CompressorTest.compress_16384/0
> > [plugin zlib (zlib/isal)]
> > [       OK ] Compressor/CompressorTest.compress_16384/0 (294 ms)
> > [ RUN      ] Compressor/CompressorTest.compress_16384/1
> > [plugin zlib (zlib/noisal)]
> > [       OK ] Compressor/CompressorTest.compress_16384/1 (1755 ms)
> > [ RUN      ] Compressor/CompressorTest.compress_16384/2
> > [plugin snappy (snappy)]
> > [       OK ] Compressor/CompressorTest.compress_16384/2 (169 ms)
> > [ RUN      ] Compressor/CompressorTest.compress_16384/3
> > [plugin zstd (zstd)]
> > [       OK ] Compressor/CompressorTest.compress_16384/3 (4528 ms)
> > 
> > It's an order of magnitude slower than zlib or snappy, which probably 
> > isn't acceptable--even if it is a bit smaller.
> > 
> > We just updated to a newer zstd the other day but I haven't been paying 
> > attention to the zstd code changes.  When I was working on this the plugin 
> > was initially also misusing the zstd API, but it was also pointed out 
> > that the size of the memset is dependent on the compression level.  
> > Maybe a different (default) choice there woudl help.
> > 
> > https://github.com/facebook/zstd/issues/408#issuecomment-252163241
> 
> thanks for the fast reply. Btrfs uses a default compression level of 3
> but i think this is the default anyway.
> 
> Does the zstd plugin of ceph already uses the mentioned
> ZSTD_resetCStream instead of creating and initializing a new one every time?

Hmm, it doesn't:

	https://github.com/ceph/ceph/blob/master/src/compressor/zstd/ZstdCompressor.h#L29

but perhaps that was because it didn't make a difference?  Might be worth 
revisiting.

> So if performance matters ceph would recommand snappy?

Yep!

sage
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