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

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Date:   Fri, 25 Aug 2017 01:35:35 +0000
From:   Nick Terrell <terrelln@...com>
To:     Joonsoo Kim <iamjoonsoo.kim@....com>
CC:     "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 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?

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.

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.

> Thanks.