Re: [PATCH 2/3] ec: make use of added aligned buffers

[Loic Dachary <loic@dachary.org>]

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Hi Janne,

See below:

On 15/09/2014 17:55, Janne Grunau wrote:
> Requiring page aligned buffers and realigning the input if necessary
> creates measurable oberhead. ceph_erasure_code_benchmark is ~30% faster
> with this change for technique=reed_sol_van,k=2,m=1.
> 
> Also prevents a misaligned buffer when bufferlist::c_str(bufferlist)
> has to allocate a new buffer to provide continuous one. See bug #9408
> 
> Signed-off-by: Janne Grunau <j@jannau.net>
> ---
>  src/erasure-code/ErasureCode.cc | 46 +++++++++++++++++++++++++----------------
>  src/erasure-code/ErasureCode.h  |  3 ++-
>  2 files changed, 30 insertions(+), 19 deletions(-)
> 
> diff --git a/src/erasure-code/ErasureCode.cc b/src/erasure-code/ErasureCode.cc
> index 5953f49..078f60b 100644
> --- a/src/erasure-code/ErasureCode.cc
> +++ b/src/erasure-code/ErasureCode.cc
> @@ -54,22 +54,38 @@ int ErasureCode::minimum_to_decode_with_cost(const set<int> &want_to_read,
>  }
>  
>  int ErasureCode::encode_prepare(const bufferlist &raw,
> -                                bufferlist *prepared) const
> +                                map<int, bufferlist> &encoded) const
>  {
>    unsigned int k = get_data_chunk_count();
>    unsigned int m = get_chunk_count() - k;
>    unsigned blocksize = get_chunk_size(raw.length());
> -  unsigned padded_length = blocksize * k;
> -  *prepared = raw;
> -  if (padded_length - raw.length() > 0) {
> -    bufferptr pad(padded_length - raw.length());
> -    pad.zero();
> -    prepared->push_back(pad);
> +  unsigned pad_len = blocksize * k - raw.length();
> +
> +  bufferlist prepared = raw;
> +
> +  if (!prepared.is_aligned()) {
> +    prepared.rebuild_aligned();
> +  }
> +
> +  for (unsigned int i = 0; i < k - !!pad_len; i++) {
> +    int chunk_index = chunk_mapping.size() > 0 ? chunk_mapping[i] : i;
> +    bufferlist &chunk = encoded[chunk_index];
> +    chunk.substr_of(prepared, i * blocksize, blocksize);
> +  }

It is possible for more than one chunk to be padding. It's a border case but... for instance with alignment = 16, k=12 and in of length 1550 you end up with two padding chunks because the blocksize is 144.

> +  if (pad_len > 0) {
> +    int chunk_index = chunk_mapping.size() > 0 ? chunk_mapping[k - 1] : k - 1;
> +    bufferlist &chunk = encoded[chunk_index];
> +    bufferptr padded(buffer::create_aligned(blocksize));
> +    raw.copy((k - 1) * blocksize, blocksize - pad_len, padded.c_str());
> +    padded.zero(blocksize - pad_len, pad_len);
> +    chunk.push_back(padded);
>    }
> -  unsigned coding_length = blocksize * m;
> -  bufferptr coding(buffer::create_page_aligned(coding_length));
> -  prepared->push_back(coding);
> -  prepared->rebuild_page_aligned();
> +  for (unsigned int i = k; i < k + m; i++) {
> +    int chunk_index = chunk_mapping.size() > 0 ? chunk_mapping[i] : i;
> +    bufferlist &chunk = encoded[chunk_index];
> +    chunk.push_back(buffer::create_aligned(blocksize));
> +  }
> +
>    return 0;
>  }
>  
> @@ -80,15 +96,9 @@ int ErasureCode::encode(const set<int> &want_to_encode,
>    unsigned int k = get_data_chunk_count();
>    unsigned int m = get_chunk_count() - k;
>    bufferlist out;
> -  int err = encode_prepare(in, &out);
> +  int err = encode_prepare(in, *encoded);
>    if (err)
>      return err;
> -  unsigned blocksize = get_chunk_size(in.length());
> -  for (unsigned int i = 0; i < k + m; i++) {
> -    int chunk_index = chunk_mapping.size() > 0 ? chunk_mapping[i] : i;
> -    bufferlist &chunk = (*encoded)[chunk_index];
> -    chunk.substr_of(out, i * blocksize, blocksize);
> -  }
>    encode_chunks(want_to_encode, encoded);
>    for (unsigned int i = 0; i < k + m; i++) {
>      if (want_to_encode.count(i) == 0)
> diff --git a/src/erasure-code/ErasureCode.h b/src/erasure-code/ErasureCode.h
> index 7aaea95..62aa383 100644
> --- a/src/erasure-code/ErasureCode.h
> +++ b/src/erasure-code/ErasureCode.h
> @@ -46,7 +46,8 @@ namespace ceph {
>                                              const map<int, int> &available,
>                                              set<int> *minimum);
>  
> -    int encode_prepare(const bufferlist &raw, bufferlist *prepared) const;
> +    int encode_prepare(const bufferlist &raw,
> +                       map<int, bufferlist> &encoded) const;
>  
>      virtual int encode(const set<int> &want_to_encode,
>                         const bufferlist &in,
> 

-- 
Loïc Dachary, Artisan Logiciel Libre


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