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  1. Lustre
  2. LU-13798

Improve direct i/o performance with multiple stripes: Submit all stripes of a DIO and then wait



    • Type: Improvement
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: Lustre 2.15.0
    • Labels:
    • Rank (Obsolete):


      The AIO implementation created in LU-4198 is able to perform at extremely high speeds because it submits multiple i/os via the direct i/o path, in a manner similar to the buffered i/o path.

      Consider the case where we do 1 MiB AIO requests with a queue depth of 64 MiB.  In this case, we submit 64 1 MiB DIO requests, and then we wait for them to complete.  (Assume we do only 64 MiB of i/o total, just for ease of conversation.)

      Critically, we submit all the i/o requests and then wait for completion.  We do not wait for completion of individual 1 MiB writes.

      Compare this now to the case where we write do a 64 MiB DIO write (or some smaller size, but > stripe size).  As LU-4198 originally noted, the performance of DIO does not scale when adding stripes.

      Consider a file with a stripe size of 1 MiB.

      This 64 MiB DIO generates 64 1 MiB writes, exactly the same as AIO with a queue depth of 64.

      Except that while the AIO request performs at ~4-5 GiB/s, the DIO request performs at ~300 MiB/s.

      This is because the DIO system submits each 1 MiB request and then waits for it:
      (Submit 1 stripe(1 MiB)) --> wait for sync, (Submit 1 stripe (1 MiB)) --> wait for sync ... etc, 64 times.

      AIO submits all of the requests and then waits, so:
      (Submit 1 stripe(1 MiB)) -> (Submit 1 stripe(1 MiB)) -> (Submit 1 stripe(1 MiB)) -> (Submit 1 stripe(1 MiB)) -> (Submit 1 stripe(1 MiB)) -> (Submit 1 stripe(1 MiB)) ->  ... (Wait for all writes to complete)

      There is no reason DIO cannot work the same way, and when we make this change, large DIO writes & reads jump in performance to the same levels as AIO with an equivalent queue depth.

      The change consists of essentially moving the waiting from the ll_direct_rw_* code up to the ll_file_io_generic layer and waiting for the completion of all submitted i/os rather than one at a time - It is a relatively simple change.

      The improvement is dramatic, from a few hundred MiB/s to roughly 5 GiB/s.

      Quick benchmark:

      mpirun -np 1 $IOR -w -r -t 256M -b 64G -o ./iorfile --posix.odirect
      Max Write: 583.03 MiB/sec (611.35 MB/sec)
      Max Read:  641.03 MiB/sec (672.17 MB/sec)
      After (w/patch):
      Max Write: 5185.96 MiB/sec (5437.87 MB/sec)
      Max Read:  5093.06 MiB/sec (5340.46 MB/sec) 

      The basic patch is relatively simple, but there are a number of additional subtleties to work out around when to do this and what sizes to submit, etc, etc.  Basic patch will be forthcoming shortly.


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              paf0186 Patrick Farrell
              paf0186 Patrick Farrell
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