linux-kernel - Re: [PATCH] zram: add zstd to the supported algorithms list

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Date:   Fri, 25 Aug 2017 10:53:01 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Nick Terrell <terrelln@...com>
Cc:     Joonsoo Kim <iamjoonsoo.kim@....com>,
        "sergey.senozhatsky.work@...il.com" 
        <sergey.senozhatsky.work@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "minchan@...nel.org" <minchan@...nel.org>,
        Yann Collet <cyan@...com>
Subject: Re: [PATCH] zram: add zstd to the supported algorithms list

On (08/25/17 01:35), Nick Terrell wrote:
> On 8/24/17, 5:49 PM, "Joonsoo Kim" <iamjoonsoo.kim@....com> wrote:
> > On Thu, Aug 24, 2017 at 09:33:54PM +0000, Nick Terrell wrote:
> > > On Thu, Aug 24, 2017 at 10:49:36AM +0900, Sergey Senozhatsky wrote:
> > > > Add ZSTD to the list of supported compression algorithms.
> > > > 
> > > > Official benchmarks [1]:
> > > 
> > > Awesome! Let me know if you need anything from me.
> > > 
> > Hello, Nick.
> > 
> > Awesome work!!!
> > 
> > Let me ask a question.
> > Zram compress and decompress a small data (a page) and your github
> > site says that using predefined dictionary would be helpful in this
> > situation. However, it seems that compression crypto API for zstd
> > doesn't use ZSTD_compress_usingDict(). Is there any plan to support
> > it?

excellent question, Joonsoo.

> I think using dictionaries in zram could be very interesting. We could for
> example, take a random sample of the RAM and use that as the dictionary
> for compression. E.g. take 32 512B samples from RAM and build a 16 KB
> dictionary (sizes may vary).
> 
> I'm not sure how you would pass a dictionary into the crypto compression
> API, but I'm sure we can make something work if dictionary compression
> proves to be beneficial enough.

a dictionaty pointer can be in `struct zstd_ctx'.


> What data have you, or anyone, used for benchmarking compression ratio and 
> speed for RAM? Since it is such a specialized application, the standard
> compression benchmarks aren't very applicable.


yeah, I thought that zstd uses dicts unconditionally.

I used my own simple minded test script:
	https://github.com/sergey-senozhatsky/zram-perf-test

it basically invokes fio with a 'static compression buffer', because
we want to have the exactly same data to be compressed when I compare
algorithms... I guess I need to improve it, somehow.

	-ss