Re: multithreaded zstd backup compression for client and server

On Wed, Mar 23, 2022 at 5:52 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> Also because the library may not be compiled with threading. A few days ago, I
> tried to rebase the original "parallel workers" patch over the COMPRESS DETAIL
> patch but then couldn't test it, even after trying various versions of the zstd
> package and trying to compile it locally. I'll try again soon...

Ah. Right, I can update the comment to mention that.

> I think you should also test the return value when setting the compress level.
> Not only because it's generally a good idea, but also because I suggested to
> support negative compression levels. Which weren't allowed before v1.3.4, and
> then the range is only defined since 1.3.6 (ZSTD_minCLevel). At some point,
> the range may have been -7..22 but now it's -131072..22.

Yeah, I was thinking that might be a good change. It would require
adjusting some other code though, because right now only compression
levels 1..22 are accepted anyhow.

> lib/compress/zstd_compress.c:int ZSTD_minCLevel(void) { return (int)-ZSTD_TARGETLENGTH_MAX; }
> lib/zstd.h:#define ZSTD_TARGETLENGTH_MAX ZSTD_BLOCKSIZE_MAX
> lib/zstd.h:#define ZSTD_BLOCKSIZE_MAX (1<<ZSTD_BLOCKSIZELOG_MAX)
> lib/zstd.h:#define ZSTD_BLOCKSIZELOG_MAX 17
> ; -1<<17
> -131072

So does that, like, compress the value by making it way bigger? :-)

--
Robert Haas
EDB: http://www.enterprisedb.com