[dpdk-dev] [PATCH] net/mlx5: poll completion queue once per a call

[Sagi Grimberg]

>>> mlx5_tx_complete() polls completion queue multiple times until it
>>> encounters an invalid entry. As Tx completions are suppressed by
>>> MLX5_TX_COMP_THRESH, it is waste of cycles to expect multiple completions
>>> in a poll. And freeing too many buffers in a call can cause high jitter.
>>> This patch improves throughput a little.
>>
>> What if the device generates burst of completions?
> mlx5 PMD suppresses completions anyway. It requests a completion per every
> MLX5_TX_COMP_THRESH Tx mbufs, not every single mbuf. So, the size of completion
> queue is even much small.

Yes I realize that, but can't the device still complete in a burst (of
unsuppressed completions)? I mean its not guaranteed that for every
txq_complete a signaled completion is pending right? What happens if
the device has inconsistent completion pacing? Can't the sw grow a
batch of completions if txq_complete will process a single completion
unconditionally?

>> Holding these completions un-reaped can theoretically cause resource stress on
>> the corresponding mempool(s).
> Can you make your point clearer? Do you think the "stress" can impact
> performance? I think stress doesn't matter unless it is depleted. And app is
> responsible for supplying enough mbufs considering the depth of all queues (max
> # of outstanding mbufs).

I might be missing something, but # of outstanding mbufs should be
relatively small as the pmd reaps every MLX5_TX_COMP_THRESH mbufs right?
Why should the pool account for the entire TX queue depth (which can
be very large)?

Is there a hard requirement documented somewhere that the application
needs to account for the entire TX queue depths for sizing its mbuf
pool?

My question is with the proposed change, doesn't this mean that the 
application might need to allocate a bigger TX mbuf pool? Because the
pmd can theoretically consume completions slower (as in multiple TX
burst calls)?

>> I totally get the need for a stopping condition, but is "loop once"
>> the best stop condition?
> Best for what?

Best condition to stop consuming TX completions. As I said, I think that
leaving TX completions un-reaped can (at least in theory) slow down the
mbuf reclamation, which impacts the application. (unless I'm not
understanding something fundamental)

>> Perhaps an adaptive budget (based on online stats) perform better?
> Please bring up any suggestion or submit a patch if any.

I was simply providing a review for the patch. I don't have the time
to come up with a better patch unfortunately, but I still think its
fair to raise a point.

> Does "budget" mean the
> threshold? If so, calculation of stats for adaptive threshold can impact single
> core performance. With multiple cores, adjusting threshold doesn't affect much.

If you look at mlx5e driver in the kernel, it maintains online stats on
its RX and TX queues. It maintain these stats mostly for adaptive
interrupt moderation control (but not only).

I was suggesting maintaining per TX queue stats on average completions
consumed for each TX burst call, and adjust the stopping condition
according to a calculated stat.