RE: [RFC PATCH v1] net/i40e: put mempool cache out of API

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>
> 
> 
> > Refer to "i40e_tx_free_bufs_avx512", this patch puts mempool cache out
> > of API to free buffers directly. There are two changes different with
> > previous version:
> > 1. change txep from "i40e_entry" to "i40e_vec_entry"
> > 2. put cache out of "mempool_bulk" API to copy buffers into it
> > directly
> >
> > Performance Test with l3fwd neon path:
> > 		with this patch
> > n1sdp:		no perforamnce change
> > amper-altra:	+4.0%
> >
> 
> 
> Thanks for RFC, appreciate your effort.
> So, as I understand - bypassing mempool put/get itself gives about 7-10%
> speedup for RX/TX on ARM platforms, correct?
It is 7% to 14% on N1SDP, 14% to 17% on Ampere machines
The current RFC needs to be converted to a patch (albeit with zero-copy mempool APIs) as it applies for a more generic use case.

> 
> About direct-rearm RX approach you propose:
> After another thought, probably it is possible to re-arrange it in a way that
> would help avoid related negatives.
Thanks for the idea.

> The basic idea as follows:
> 
> 1. Make RXQ sw_ring visible and accessible by 'attached' TX queues.
>     Also make sw_ring de-coupled from RXQ itself, i.e:
>     when RXQ is stopped or even destroyed, related sw_ring may still
>     exist (probably ref-counter or RCU would be sufficient here).
>     All that means we need a common layout/api for rxq_sw_ring
>     and PMDs that would like to support direct-rearming will have to
>     use/obey it.
This would mean, we will have additional for loop to fill the descriptors on the RX side.
May be we could keep the queue when the RXQ is stopped, and free it only when RXQ is destroyed. Don't have to allocate one more if the RXQ is started back again? This means, the mbufs will be in the sw_ring if the RXQ is stopped but not destroyed (same as what you have mentioned below).

> 
> 2. Make RXQ sw_ring 'direct' rearming driven by TXQ itself, i.e:
>     at txq_free_bufs() try to store released mbufs inside attached
>     sw_ring directly. If there is no attached sw_ring, or not enough
>     free space in it - continue with mempool_put() as usual.
>     Note that actual arming of HW RXDs still remains responsibility
What problems do you see if we arm the HW RXDs (assuming that we do not have to support this across multiple devices)?

>     of RX code-path:
>     rxq_rearm(rxq) {
>       ...
>       - check are there are N already filled entries inside rxq_sw_ring.
>         if not, populate them from mempool (usual mempool_get()).
>       - arm related RXDs and mark these sw_ring entries as managed by HW.
>       ...
>     }
> 
> 
> So rxq_sw_ring will serve two purposes:
> - track mbufs that are managed by HW (that what it does now)
> - private (per RXQ) mbuf cache
> 
> Now, if TXQ is stopped while RXQ is running - no extra synchronization is
> required, RXQ would just use
> mempool_get() to rearm its sw_ring itself.
There would be some synchronization required which tells the data plane threads not to access the TXQ before the TXQ is stopped. Other than this, no extra synchronization is required.

For the current patch, we could use a similar approach. i.e. when TXQ is stopped, it does not free the mbufs from sw_ring, we just let the RXQ consume all the mbufs from TX side sw_ring till it is empty.

> 
> If RXQ is stopped while TXQ is still running - TXQ can still continue to populate
> related sw_ring till it gets full.
> Then it will continue with mempool_put() as usual.
> Of-course it means that user who wants to use this feature should probably
> account some extra mbufs for such case, or might be rxq_sw_ring can have
> enable/disable flag to mitigate such situation.
> 
> As another benefit here - such approach makes possible to use several TXQs
> (even from different devices) to rearm same RXQ.
Being able to use several TXQs is a practical use case. But, I am not sure if different devices in the same server is a practical scenario that we need to address.

> 
> Have to say, that I am still not sure that 10% RX/TX improvement is worth
> bypassing mempool completely and introducing all this extra complexity in
> RX/TX path.
It is more, clarified above.

> But, if we'll still decide to go ahead with direct-rearming, this re-arrangement, I
> think, should help to keep things clear and avoid introducing new limitations in
> existing functionality.
> 
> WDYT?
I had another idea. This requires the application to change, which might be ok given that it is new feature/optimization.

We could expose a new API to the application which allows RX side to take buffers from TX side. The application would call this additional API just before calling the eth_rx_burst API.

The advantages I see with this approach is:
1) The static mapping of the ports is not required. The mapping is dynamic. This allows for the application to make decisions based on current conditions in the data plane.
2) Code is simple, because it does not have to check for many conditions. The application can make better decisions as it knows the scenario well. Not all applications have to take the hit of conditionals.
2) The existing synchronization used by the applications (to stop RXQ/TXQ) is sufficient. No new synchronization required.

The disadvantage is that it is another function pointer call which will reduce some performance.

> 
> Konstantin
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