11/07/2022 07:19, Honnappa Nagarahalli пишет:
<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.
Yes, rx will go though avaiable entries in sw_ring and fill actual HW
descriptors.
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).
My thought here was - yes sw_ring can stay at queue stop
(even probably at destroy) to avoid related TXQ operation abortion.
Though actual HW desc ring will be de-init/destroyed as usual.
That's why I think we need to decouple sw_ring from queue and
it's HW ring.
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)?
Same as visa-versa scenario:
- RXQ might be already stopped, while TXQ still running.
- Also it doesn't sound right from design point of view to allow
TX code to manipulate RX HW queue directly and visa-versa.
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.
That sounds like a very good idea to me.
I think it will allow to avoid all short-comings that are
present in original approach.
Again, if we like we can even allow user to feed RXQ from some other
sources (dropped packets).
Just to confirm that we are talking about the same thing:
1. rte_ethdev will have an API for the user to get sw_ring handle
from the RXQ, something like:
struct rxq_sw_ring *
rte_eth_rxq_get_sw_ring(uint16_t port_id, uint16_t rxq_id);
We probbly can even keep 'struct rxq_sw_ring' as internal one,
and use it as a handle.
2. rte_ethdev will have an API that will ask TXQ to free mbufs
and fill given rxq_sw_ring:
int
rte_eth_txq_fill_sw_ring(uint16_t port_id, uint16_t txq_id, struct *
rxq_sw_ring);
So the app code will look something like:
rxq_sw_ring = rxq_get_sw_ring(rx_port, rxq);
...
while (...) {
n = eth_tx_burst(tx_port, txq, pkts, N);
...
eth_txq_fill_sw_ring(tx_port, txq, rxq_sw_ring);
n = eth_rx_burst(rx_port, rxq, pkts, N);
...
}
Did I get your idea right?
The disadvantage is that it is another function pointer call which will reduce
some performance.
Konstantin
