linux-kernel - Re: [PATCH 1/1] lz4: Implement lz4 with dynamic offset length.

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Date:   Wed, 4 Apr 2018 10:40:23 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Vaneet Narang <v.narang@...sung.com>
Cc:     Maninder Singh <maninder1.s@...sung.com>,
        "sergey.senozhatsky.work@...il.com" 
        <sergey.senozhatsky.work@...il.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "ngupta@...are.org" <ngupta@...are.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        PANKAJ MISHRA <pankaj.m@...sung.com>,
        AMIT SAHRAWAT <a.sahrawat@...sung.com>
Subject: Re: [PATCH 1/1] lz4: Implement lz4 with dynamic offset length.

On (04/03/18 19:13), Vaneet Narang wrote:
> Hi Sergey,
> 
> >You shrink a 2 bytes offset down to a 1 byte offset, thus you enforce that
> 2 Byte offset is not shrinked to 1 byte, Its only 1 bit is reserved out of
> 16 bits of offset. So only 15 Bits can be used to store offset value.

Yes, you are right. My bad, was thinking about something else.

> >'page should be less than 32KB', which I'm sure will be confusing. 
> lz4_dyn will work on bigger data length(> 32k) but in that case compression
> ratio may not be better than LZ4. This is same as LZ4 compressing data more 
> than 64K (16Bits). LZ4 can't store offset more than 64K similarly
> LZ4 dyn can't store offset more than 32K.

Then drop that `if PAGE_SIZE' thing. I'd rather do that stuff internally
in lz4... if it needed at all.

> >And you
> >rely on lz4_dyn users to do the right thing - namely, to use that 'nice'
> >`#if (PAGE_SIZE < (32 * KB))'. 
> They don't need to add this code

Then drop it.

> >Apart from that, lz4_dyn supports only data
> >in up to page_size chunks. Suppose my system has page_size of less than 32K,
> >so I legitimately can enable lz4_dyn, but suppose that I will use it
> >somewhere where I don't work with page_size-d chunks. Will I able to just
> >do tfm->compress(src, sz) on random buffers? The whole thing looks to be
> >quite fragile.
> No thats not true, lz4_dyn can work for random buffers and it need not be 
> of page size chunks. There is no difference in Lz4 and Lz4 dyn working. 

You are right.

	-ss