From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Andrey Ignatov <rdna@apple.com>, dev@dpdk.org
Cc: Chenbo Xia <chenbox@nvidia.com>, Wei Shen <wshen0123@apple.com>
Subject: Re: [PATCH] vhost: optimize mbuf allocation in virtio Tx packed path
Date: Wed, 3 Apr 2024 12:19:46 +0200 [thread overview]
Message-ID: <98c1642c-c32d-46b5-a34c-4bfcc530905f@redhat.com> (raw)
In-Reply-To: <20240328233338.56544-1-rdna@apple.com>
On 3/29/24 00:33, Andrey Ignatov wrote:
> Currently virtio_dev_tx_packed() always allocates requested @count of
> packets, no matter how many packets are really available on the virtio
> Tx ring. Later it has to free all packets it didn't use and if, for
> example, there were zero available packets on the ring, then all @count
> mbufs would be allocated just to be freed afterwards.
>
> This wastes CPU cycles since rte_pktmbuf_alloc_bulk() /
> rte_pktmbuf_free_bulk() do quite a lot of work.
>
> Optimize it by using the same idea as the virtio_dev_tx_split() uses on
> the Tx split path: estimate the number of available entries on the ring
> and allocate only that number of mbufs.
>
> On the split path it's pretty easy to estimate.
>
> On the packed path it's more work since it requires checking flags for
> up to @count of descriptors. Still it's much less expensive than the
> alloc/free pair.
>
> The new get_nb_avail_entries_packed() function doesn't change how
> virtio_dev_tx_packed() works with regard to memory barriers since the
> barrier between checking flags and other descriptor fields is still in
> place later in virtio_dev_tx_batch_packed() and
> virtio_dev_tx_single_packed().
>
> The difference for a guest transmitting ~17Gbps with MTU 1500 on a `perf
> record` / `perf report` (on lower pps the savings will be bigger):
>
> * Before the change:
>
> Samples: 18K of event 'cycles:P', Event count (approx.): 19206831288
> Children Self Pid:Command
> - 100.00% 100.00% 798808:dpdk-worker1
> <... skip ...>
> - 99.09% pkt_burst_io_forward
> - 90.26% common_fwd_stream_receive
> - 90.04% rte_eth_rx_burst
> - 75.53% eth_vhost_rx
> - 74.29% rte_vhost_dequeue_burst
> - 71.48% virtio_dev_tx_packed_compliant
> + 17.11% rte_pktmbuf_alloc_bulk
> + 11.80% rte_pktmbuf_free_bulk
> + 2.11% vhost_user_inject_irq
> 0.75% rte_pktmbuf_reset
> 0.53% __rte_pktmbuf_free_seg_via_array
> 0.88% vhost_queue_stats_update
> + 13.66% mlx5_rx_burst_vec
> + 8.69% common_fwd_stream_transmit
>
> * After:
>
> Samples: 18K of event 'cycles:P', Event count (approx.): 19225310840
> Children Self Pid:Command
> - 100.00% 100.00% 859754:dpdk-worker1
> <... skip ...>
> - 98.61% pkt_burst_io_forward
> - 86.29% common_fwd_stream_receive
> - 85.84% rte_eth_rx_burst
> - 61.94% eth_vhost_rx
> - 60.05% rte_vhost_dequeue_burst
> - 55.98% virtio_dev_tx_packed_compliant
> + 3.43% rte_pktmbuf_alloc_bulk
> + 2.50% vhost_user_inject_irq
> 1.17% vhost_queue_stats_update
> 0.76% rte_rwlock_read_unlock
> 0.54% rte_rwlock_read_trylock
> + 22.21% mlx5_rx_burst_vec
> + 12.00% common_fwd_stream_transmit
>
> It can be seen that virtio_dev_tx_packed_compliant() goes from 71.48% to
> 55.98% with rte_pktmbuf_alloc_bulk() going from 17.11% to 3.43% and
> rte_pktmbuf_free_bulk() going away completely.
>
> Signed-off-by: Andrey Ignatov <rdna@apple.com>
> ---
> lib/vhost/virtio_net.c | 33 +++++++++++++++++++++++++++++++++
> 1 file changed, 33 insertions(+)
>
Thanks for the contribution and the detailed commit message.
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Maxime
prev parent reply other threads:[~2024-04-03 10:19 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-28 23:33 Andrey Ignatov
2024-03-28 23:44 ` Stephen Hemminger
2024-03-29 0:10 ` Andrey Ignatov
2024-03-29 2:53 ` Stephen Hemminger
2024-03-29 13:04 ` Maxime Coquelin
2024-03-29 13:42 ` The effect of inlining Morten Brørup
2024-03-29 20:26 ` Tyler Retzlaff
2024-04-01 15:20 ` Mattias Rönnblom
2024-04-03 16:01 ` Morten Brørup
2024-04-03 10:19 ` Maxime Coquelin [this message]
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