[PATCH 00/11] makedumpfile: Add zstd support for makedumpfile

[HAGIO KAZUHITO(萩尾 一仁)]

Hi Tao Liu,

-----Original Message-----
> Hi Kazu,
> 
> Thanks for reviewing the patchset!
> 
> On Tue, Sep 14, 2021 at 07:04:24AM +0000, HAGIO KAZUHITO(萩尾 一仁) wrote:
> > Hi Tao Liu,
> >
> > Thanks for the patchset!
> >
> > -----Original Message-----
> > > This patch set adds ZSTD compression support to makedumpfile. With ZSTD compression
> > > support, the vmcore dump size and time consumption can have a better balance than
> > > zlib/lzo/snappy.
> > >
> > > How to build:
> > >
> > >   Build using make:
> > >     $ make USEZSTD=on
> > >
> > > Performance Comparison:
> > >
> > >   How to measure
> > >
> > >     I took a x86_64 machine which had 4T memory, and the compression level
> > >     range from (-3 to 4) for ZSTD, as well as zlib/lzo/snappy compression.
> > >     All testing was done by makedumpfile single thread mode.
> > >
> > >     As for compression performance testing, in order to avoid the performance
> > >     bottle neck of disk I/O, I used the following makedumpfile cmd, which took
> > >     lzo compression as an example. "--dry-run" will not write any data to disk,
> > >     "--show-stat" will output the vmcore size after compression, and the time
> > >     consumption can be collected from the output logs.
> > >
> > >     $ makedumpfile -d 0 -l /proc/kcore vmcore --dry-run --show-stat
> > >
> > >
> > >     As for decompression performance testing, I only tested the (-d 31) case,
> > >     because the vmcore size of (-d 0) case is too big to fit in the disk, in
> > >     addtion, to read a oversized file from disk will encounter the disk I/O
> > >     bottle neck.
> > >
> > >     I triggered a kernel crash and collected a vmcore. Then I converted the
> > >     vmcore into specific compression format using the following makedumpfile
> > >     cmd, which would get a lzo format vmcore as an example:
> > >
> > >     $ makedumpfile -l vmcore vmcore.lzo
> > >
> > >     After all vmcores were ready, I used the following cmd to perform the
> > >     decompression, the time consumption can be collected from the logs.
> > >
> > >     $ makedumpfile -F vmcore.lzo --dry-run --show-stat
> > >
> > >
> > >   Result charts
> > >
> > >     For compression:
> > >
> > >             makedumpfile -d31			|  makedumpfile -d0
> > >             Compression time	vmcore size	|  Compression time  vmcore size
> > >     zstd-3  325.516446	        5285179595	|  8205.452248	     51715430204
> > >     zstd-2  332.069432	        5319726604	|  8057.381371	     51732062793
> > >     zstd-1  309.942773	        5730516274	|  8138.060786	     52136191571
> > >     zstd0   439.773076	        4673859661	|  8873.059963	     50993669657
> > >     zstd1   406.68036	        4700959521	|  8259.417132	     51036900055
> > >     zstd2   397.195643	        4699263608	|  8230.308291	     51030410942
> > >     zstd3   436.491632	        4673306398	|  8803.970103	     51043393637
> > >     zstd4   543.363928	        4668419304	|  8991.240244	     51058088514
> > >     zlib    561.217381	        8514803195      | 14381.755611	     78199283893
> > >     lzo	    248.175953	       16696411879	|  6057.528781	     90020895741
> > >     snappy  231.868312	       11782236674	|  5290.919894	    245661288355
> > >
> > >     For decompression:
> > >
> > >             makedumpfile -d31
> > >             decompress time	   vmcore size
> > >     zstd-3	477.543396	       5289373448
> > >     zstd-2	478.034534	       5327454123
> > >     zstd-1	459.066807	       5748037931
> > >     zstd0	561.687525	       4680009013
> > >     zstd1	547.248917	       4706358547
> > >     zstd2	544.219758	       4704780719
> > >     zstd3	555.726343	       4680009013
> > >     zstd4	558.031721	       4675545933
> > >     zlib	630.965426	       8555376229
> > >     lzo	    	427.292107	      16849457649
> > >     snappy	446.542806	      11841407957
> > >
> > >   Discussion
> > >
> > >     For zstd range from -3 to 4, compression level 2 (ZSTD_dfast) has
> > >     the best time consumption and vmcore dump size balance.
> >
> > Do you have a result of -d 1 compression test?  I think -d 0 is not
> > practical, I would like to see a -d 1 result of such a large vmcore.
> >
> 
> No, I haven't tested the -d 1 case. I have returned the machine which used
> for performance testing, I will borrow and test on it again, please wait for
> a while...

Thanks, it would be helpful.

> 
> > And just out of curiosity, what version of zstd are you using?
> > When I tested zstd last time, compression level 1 was faster than 2, iirc.
> >
> 
> The OS running on the machine is fedora34, I used its default zstd package, whose
> version is v1.4.9.

Thanks for the info.

> 
> > btw, ZSTD_dfast is an enum of ZSTD_strategy, not for compression level?
> 
> Yes, it's enum of ZSTD_strategy [1].

ok, so it'll have to be replaced with "2" to avoid confusion.

> 
> [1]: https://zstd.docsforge.com/dev/api-documentation/#advanced-compression-api-requires-v140
> 
> Thanks,
> Tao Liu
> 
> > (no need to update for now, I will review later)

The series almost looks good to me (though I will merge those into a patch),
just two questions are:
- whether 2 is the best balanced compression level,
- how much ZSTD_decompressDCtx() is faster than the current ZSTD_decompress().

I'll evaluate these, but it would be helpful if you could do some, too.

I think that compression time and ratio will vary with the data, it'll be
better to use some real data, I'm looking for it.. kernel source or something.

Thanks,
Kazu