[Skiboot-stable] [PATCH v3] hw/phb4: Tune GPU direct performance on witherspoon in PCI mode

[Oliver O'Halloran]

On Wed, Mar 25, 2020 at 9:41 PM Frederic Barrat <fbarrat at linux.ibm.com> wrote:
>
> Good GPU direct performance on witherspoon, with a Mellanox adapter
> on the shared slot, requires to reallocate some dma engines within
> PEC2, "stealing" some from PHB4&5 and giving extras to PHB3. It's
> currently done when using CAPI mode. But the same is true if the
> adapter stays in PCI mode.
>
> In preparation for upcoming versions of MOFED, which may not use CAPI
> mode, this patch reallocates dma engines even in PCI mode for a series
> of Mellanox adapters that can be used with GPU direct, on witherspoon
> and on the shared slot only.
>
> The loss of dma engines for PHB4&5 on witherspoon has not shown
> problems in testing, as well as in current deployments where CAPI mode
> is used.
>
> Here is a comparison of the bandwidth numbers seen with the PHB in PCI
> mode (no CAPI) with and without this patch. Variations on smaller
> packet sizes can be attributed to jitter and are not that meaningful.
>
>  # OSU MPI-CUDA Bi-Directional Bandwidth Test v5.6.1
>  # Send Buffer on DEVICE (D) and Receive Buffer on DEVICE (D)
>  # Size      Bandwidth (MB/s)       Bandwidth (MB/s)
>  #              with patch           without patch
> 1                       1.29              1.48
> 2                       2.66              3.04
> 4                       5.34              5.93
> 8                      10.68             11.86
> 16                     21.39             23.71
> 32                     42.78             49.15
> 64                     85.43             97.67
> 128                   170.82            196.64
> 256                   385.47            383.02
> 512                   774.68            755.54
> 1024                 1535.14           1495.30
> 2048                 2599.31           2561.60
> 4096                 5192.31           5092.47
> 8192                 9930.30           9566.90
> 16384               18189.81          16803.42
> 32768               24671.48          21383.57
> 65536               28977.71          24104.50
> 131072              31110.55          25858.95
> 262144              32180.64          26470.61
> 524288              32842.23          26961.93
> 1048576             33184.87          27217.38
> 2097152             33342.67          27338.08
>
> Signed-off-by: Frederic Barrat <fbarrat at linux.ibm.com>
> Cc: skiboot-stable at lists.ozlabs.org # skiboot-op940.x

Thanks, merged as e876514b3773