[tahoe-lafs-trac-stream] [tahoe-lafs] #1398: make docs/performance.rst more precise and accurate

[tahoe-lafs]

#1398: make docs/performance.rst more precise and accurate
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     Reporter:  zooko    |      Owner:  warner
         Type:  defect   |     Status:  new
     Priority:  major    |  Milestone:  1.9.0
    Component:           |    Version:  1.8.2
  documentation          |   Keywords:  scalability docs performance memory
   Resolution:           |  large review-needed
Launchpad Bug:           |
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Comment (by warner):

 I think I *was* thinking of encryption/upload in that original
 description, not verify: good catch.


 * '''publishing immutable when the file is already uploaded''': our
   Uploader doesn't short-circuit the upload when it discovers that
   enough shares are already in place. The upload sequence runs
   (including encryption and erasure-coding of each share), but the
   results are discarded. (this is arguably an inefficiency, but I'm not
   convinced the additional code paths would be worth it). So the memory
   footprint is the same as for not-already-uploaded
 * '''publishing immutable when the file is not uploaded''': do you want
   to include the constant multiple of memory footprint that results from
   the breadth/overlap of the pipeline? Specifically, we hold both
   plaintext and ciphertext in memory briefly, and then hold both
   ciphertext and encoded shares simultaneously. That makes it more like
   {{{(1+N/K)*S}}}.
 * '''publishing mutable file''': network is the same as for immutable:
   {{{N/K*A}}}, not just {{{A}}} (SDMF has one *segment*, not one
   *share*). Also, this should probably refer specifically to SDMF, since
   MDMF has different characteristics.

 Other than that, it looks accurate. Some entries are missing memory
 footprints, and most memory footprints could take the constant-multiple
 {{{(1+N/K)*S}}} overlap into account if you want to get that detailed.

-- 
Ticket URL: <http://tahoe-lafs.org/trac/tahoe-lafs/ticket/1398#comment:6>
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