Re: v2 aligned buffer changes for erasure codes

[Loic Dachary <loic@dachary.org>]

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Hi Andreas,

The per_chunk_alignment addresses a backward compatible change in the way they are calculated. The problem was that the initial calculation lead to oversized chunks. The long explanation is at https://github.com/ceph/ceph/commit/c7daaaf5e63d0bd1d444385e62611fe276f6ce29

Please let me know if you see something wrong :-)

Cheers

On 18/09/2014 14:34, Andreas Joachim Peters wrote:
> Hi Janne/Loic, 
> there is more confusion atleast on my side ...
> 
> I had now a look at the jerasure plug-in and I am now slightly confused why you have two ways to return in get_alignment ... one is as I assume and another one is "per_chunk_alignment" ... what should the function return Loic?
> 
> Cheers Andreas.
> ________________________________________
> From: ceph-devel-owner@vger.kernel.org [ceph-devel-owner@vger.kernel.org] on behalf of Andreas Joachim Peters [Andreas.Joachim.Peters@cern.ch]
> Sent: 18 September 2014 14:18
> To: Janne Grunau; ceph-devel@vger.kernel.org
> Subject: RE: v2 aligned buffer changes for erasure codes
> 
> Hi Janne,
> => (src/erasure-code/isa/README claims it needs 16*k byte aligned buffers
> 
> I should update the README since it is misleading ... it should say 8*k or 16*k byte aligned chunk size depending on the compiler/platform used, it is not the alignment of the allocated buffer addresses.The get_alignment in the plug-in function is used to compute the chunk size for the encoding (as I said not the start address alignment).
> 
> If you pass k buffers for decoding each buffer should be aligned at least to 16 or as you pointed out better 32 bytes.
> 
> For encoding there is normally a single buffer split 'virtually' into k pieces. To make all pieces starting at an aligned address one needs to align the chunk size to e.g. 16*k. For the best possible performance on all platforms we should change the get_alignment function in the ISA plug-in to return 32*k if there are no other objections ?!?!
> 
> Cheers Andreas.
> ________________________________________
> From: ceph-devel-owner@vger.kernel.org [ceph-devel-owner@vger.kernel.org] on behalf of Janne Grunau [j@jannau.net]
> Sent: 18 September 2014 12:33
> To: ceph-devel@vger.kernel.org
> Subject: v2 aligned buffer changes for erasure codes
> 
> Hi,
> 
> following a is an updated patchset. It passes now make check in src
> 
> It has following changes:
>  * use 32-byte alignment since the isa plugin use AVX2
>    (src/erasure-code/isa/README claims it needs 16*k byte aligned buffers
>    but I can't see a reason why it would need more than 32-bytes
>  * ErasureCode::encode_prepare() handles more than one chunk with padding
> 
> cheers
> 
> Janne
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-- 
Loïc Dachary, Artisan Logiciel Libre


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