On Wed, Jun 26, 2024 at 10:16:06PM +0200, Morten Brørup wrote:
> > From: Mattias Rönnblom [mailto:hof...@lysator.liu.se]
> > Sent: Wednesday, 26 June 2024 20.48
> >
> > On Wed, Jun 26, 2024 at 05:24:04PM +0200, Maxime Coquelin wrote:
> > >
> > >
> > > On 6/26/24 16:58, Stephen Hemminger wrote:
> > > > On Wed, 26 Jun 2024 10:37:31 +0200
> > > > Maxime Coquelin <maxime.coque...@redhat.com> wrote:
> > > >
> > > > > On 6/25/24 21:27, Mattias Rönnblom wrote:
> > > > > > On Tue, Jun 25, 2024 at 05:29:35PM +0200, Maxime Coquelin wrote:
> > > > > > > Hi Mattias,
> > > > > > >
> > > > > > > On 6/20/24 19:57, Mattias Rönnblom wrote:
> > > > > > > > This patch set make DPDK library, driver, and application
> > code use the
> > > > > > > > compiler/libc memcpy() by default when functions in
> > <rte_memcpy.h> are
> > > > > > > > invoked.
> > > > > > > >
> > > > > > > > The various custom DPDK rte_memcpy() implementations may be
> > retained
> > > > > > > > by means of a build-time option.
> > > > > > > >
> > > > > > > > This patch set only make a difference on x86, PPC and ARM.
> > Loongarch
> > > > > > > > and RISCV already used compiler/libc memcpy().
> > > > > > >
> > > > > > > It indeed makes a difference on x86!
> > > > > > >
> > > > > > > Just tested latest main with and without your series on
> > > > > > > Intel(R) Xeon(R) Gold 6438N.
> > > > > > >
> > > > > > > The test is a simple IO loop between a Vhost PMD and a Virtio-
> > user PMD:
> > > > > > > # dpdk-testpmd -l 4-6 --file-prefix=virtio1 --no-pci --vdev
> > 'net_virtio_user0,mac=00:01:02:03:04:05,path=./vhost-
> > net,server=1,mrg_rxbuf=1,in_order=1'
> > > > > > > --single-file-segments -- -i
> > > > > > > testpmd> start
> > > > > > >
> > > > > > > # dpdk-testpmd -l 8-10 --file-prefix=vhost1 --no-pci --vdev
> > > > > > > 'net_vhost0,iface=vhost-net,client=1' --single-file-segments
> > -- -i
> > > > > > > testpmd> start tx_first 32
> > > > > > >
> > > > > > > Latest main: 14.5Mpps
> > > > > > > Latest main + this series: 10Mpps
> > > > > >
> > > > > > I ran the above benchmark on my Raptor Lake desktop (locked to
> > 3,2
> > > > > > GHz). GCC 12.3.0.
> > > > > >
> > > > > > Core use_cc_memcpy Mpps
> > > > > > E false 9.5
> > > > > > E true 9.7
> > > > > > P false 16.4
> > > > > > P true 13.5
> > > > > >
> > > > > > On the P-cores, there's a significant performance regression,
> > although
> > > > > > not as bad as the one you see on your Sapphire Rapids Xeon. On
> > the
> > > > > > E-cores, there's actually a slight performance gain.
> > > > > >
> > > > > > The virtio PMD does not directly invoke rte_memcpy() or anything
> > else
> > > > > > from <rte_memcpy.h>, but rather use memcpy(), so I'm not sure I
> > > > > > understand what's going on here. Does the virtio driver delegate
> > some
> > > > > > performance-critical task to some module that in turns uses
> > > > > > rte_memcpy()?
> > > > >
> > > > > This is because Vhost is the bottleneck here, not Virtio driver.
> > > > > Indeed, the virtqueues memory belongs to the Virtio driver and the
> > > > > descriptors buffers are Virtio's mbufs, so not much memcpy's are
> > done
> > > > > there.
> > > > >
> > > > > Vhost however, is a heavy memcpy user, as all the descriptors
> > buffers
> > > > > are copied to/from its mbufs.
> > > >
> > > > Would be good to now the size (if small it is inlining that matters,
> > or
> > > > maybe alignment matters), and have test results for multiple
> > compiler versions.
> > > > Ideally, feed results back and update Gcc and Clang.
> > >
> > > I was testing with GCC 11 on RHEL-9:
> > > gcc (GCC) 11.4.1 20231218 (Red Hat 11.4.1-3)
> > >
> > > I was using the default one, 64B packets.
> > >
> > > I don't have time to perform these tests, but if you are willing to do
> > > it I'll be happy to review the results.
> > >
> > > > DPDK doesn't need to be in the optimize C library space.
> > >
> > > Certainly, but we already have an optimized version currently, so not
> > > much to do now on our side. When C libraries implementations will be
> > on
> > > par, we should definitely use them by default.
> > >
> >
> > I think it's not so much about optimized versus non-optimized at this
> > point. It's just that cc/libc memcpy sometimes performs better than
> > RTE memcpy, and sometimes doesn't.
> >
> > For virtio, a single memory copy in
> > lib/vhost/virtio_net.c:do_data_copy_enqueue()
> > is responsible for >95% of the performance regression introduced by
> > the cc memcpy patch for small packets on Intel P-cores.
> >
> > I'm not so sure this performance regression will go away in newer
> > compilers. PGO would certainly help, but PGO is a hassle.
> >
> > One way to fix this issue would be to introduce a custom,
> > memcpy()-based packet copying routine. I tried the below patch, with
> > the following results:
> >
> > Raptor Lake @ 3,2 GHz
> > GCC 12
> >
> > 64 bytes packets
> > Core Mode Mpps
> > ----------------------------
> > E RTE memcpy 9.5
> > E cc memcpy 9.7
> > E cc memcpy+pktcpy 9.0
> >
> > P RTE memcpy 16.4
> > P cc memcpy 13.5
> > P cc memcpy+pktcpy 16.2
> >
> > 1500 bytes
> > Core Mode Mpps
> > ----------------------------
> > P RTE memcpy 5.8
> > P cc memcpy 5.9
> > P cc memcpy+pktcpy 5.9
> >
> > As you can see, most of the regression is eliminated, at the cost of
> > worse E-core performance. I didn't look at the generated code, but one
> > could suspect heavy use of wide SIMD is to blame, which E-cores don't
> > necessarily benefit from.
> >
> > The below prototype assumes the source and destination buffers are
> > 16-byte aligned. Does that always hold?
>
> Perhaps always for this specific function; I don't know.
> Not generally *always*, but I guess in many cases packet copies would have
> 64-byte aligned pointers, but not sizes.
> Unless explicitly stated by the developer, it is unsafe to make assumptions
> about alignment.
>
I meant always (for every packet) in DPDK virtio net.
> A future rte_memcpy() function might take flags with explicit alignment
> information for optimized copying, as was part of my non-temporal memcpy().
> (The development on this is still on hold.)
>
There is already a mechanism to express alignment in GCC-compatible
compilers. See the below patch.
> >
> > I'm sure one could further improve performance using context-specific
> > information, such as packets always being >= 64 bytes. One could also
> > consider having special cases, maybe for 64 bytes and MTU-sized
> > packets. Such are always a hassle when you try to characterize
> > performance though.
>
> Absolutely!
>
> This got me thinking:
> These tests are run with 64 byte packets only.
> Perhaps branch prediction pollutes the results, by optimizing branches in the
> copy routine for all packets being 64 byte.
>
> You really should be testing with IMIX or random packet sizes.
>
Sure, and it should also be a real app, not testpmd, on top of virtio.
> In my experience, most internet packets are large (> 1024 byte, but not 1514
> byte due to QUIC's conservative max packet size), closely followed by 64 byte
> (excl. any VLAN tags) packets; only the minority of packets are medium size.
>
> >
> > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> > index 370402d849..7b595a6622 100644
> > --- a/lib/vhost/virtio_net.c
> > +++ b/lib/vhost/virtio_net.c
> > @@ -231,6 +231,26 @@ vhost_async_dma_check_completed(struct virtio_net
> > *dev, int16_t dma_id, uint16_t
> > return nr_copies;
> > }
> >
> > +static inline void
> > +pktcpy(void *restrict in_dst, const void *restrict in_src, size_t len)
> > +{
> > + void *dst = __builtin_assume_aligned(in_dst, 16);
> > + const void *src = __builtin_assume_aligned(in_src, 16);
> > +
> > + if (len <= 256) {
> > + size_t left;
> > +
> > + for (left = len; left >= 32; left -= 32) {
> > + memcpy(dst, src, 32);
> > + dst = RTE_PTR_ADD(dst, 32);
> > + src = RTE_PTR_ADD(src, 32);
> > + }
> > +
> > + memcpy(dst, src, left);
> > + } else
> > + memcpy(dst, src, len);
> > +}
> > +
> > static inline void
> > do_data_copy_enqueue(struct virtio_net *dev, struct vhost_virtqueue
> > *vq)
> > __rte_shared_locks_required(&vq->iotlb_lock)
> > @@ -240,7 +260,7 @@ do_data_copy_enqueue(struct virtio_net *dev, struct
> > vhost_virtqueue *vq)
> > int i;
> >
> > for (i = 0; i < count; i++) {
> > - rte_memcpy(elem[i].dst, elem[i].src, elem[i].len);
> > + pktcpy(elem[i].dst, elem[i].src, elem[i].len);
> > vhost_log_cache_write_iova(dev, vq, elem[i].log_addr,
> > elem[i].len);
> > PRINT_PACKET(dev, (uintptr_t)elem[i].dst, elem[i].len, 0);
> > @@ -257,7 +277,7 @@ do_data_copy_dequeue(struct vhost_virtqueue *vq)
> > int i;
> >
> > for (i = 0; i < count; i++)
> > - rte_memcpy(elem[i].dst, elem[i].src, elem[i].len);
> > + pktcpy(elem[i].dst, elem[i].src, elem[i].len);
> >
> > vq->batch_copy_nb_elems = 0;
> > }
> >
> >
> >
> > > Maxime
> > >
