RE: [PATCH v4] net: fix checksum with unaligned buffer

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> Sent: Monday, 27 June 2022 19.23
> 
> On 2022-06-27 15:22, Morten Brørup wrote:
> >> From: Emil Berg [mailto:emil.berg@ericsson.com]
> >> Sent: Monday, 27 June 2022 14.51
> >>
> >>> From: Emil Berg
> >>> Sent: den 27 juni 2022 14:46
> >>>
> >>>> From: Mattias Rönnblom <hofors@lysator.liu.se>
> >>>> Sent: den 27 juni 2022 14:28
> >>>>
> >>>> On 2022-06-23 14:51, Morten Brørup wrote:
> >>>>>> From: Morten Brørup [mailto:mb@smartsharesystems.com]
> >>>>>> Sent: Thursday, 23 June 2022 14.39
> >>>>>>
> >>>>>> With this patch, the checksum can be calculated on an unaligned
> >> buffer.
> >>>>>> I.e. the buf parameter is no longer required to be 16 bit
> >> aligned.
> >>>>>>
> >>>>>> The checksum is still calculated using a 16 bit aligned pointer,
> >> so
> >>>>>> the compiler can auto-vectorize the function's inner loop.
> >>>>>>
> >>>>>> When the buffer is unaligned, the first byte of the buffer is
> >>>>>> handled separately. Furthermore, the calculated checksum of the
> >>>>>> buffer is byte shifted before being added to the initial
> >> checksum,
> >>>>>> to compensate for the checksum having been calculated on the
> >> buffer
> >>>>>> shifted by one byte.
> >>>>>>
> >>>>>> v4:
> >>>>>> * Add copyright notice.
> >>>>>> * Include stdbool.h (Emil Berg).
> >>>>>> * Use RTE_PTR_ADD (Emil Berg).
> >>>>>> * Fix one more typo in commit message. Is 'unligned' even a
> >> word?
> >>>>>> v3:
> >>>>>> * Remove braces from single statement block.
> >>>>>> * Fix typo in commit message.
> >>>>>> v2:
> >>>>>> * Do not assume that the buffer is part of an aligned packet
> >> buffer.
> >>>>>>
> >>>>>> Bugzilla ID: 1035
> >>>>>> Cc: stable@dpdk.org
> >>>>>>
> >>>>>> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>

[...]

> >>>>
> >>>> The compiler will be able to auto vectorize even unaligned
> >> accesses,
> >>>> just with different instructions. From what I can tell, there's no
> >>>> performance impact, at least not on the x86_64 systems I tried on.
> >>>>
> >>>> I think you should remove the first special case conditional and
> >> use
> >>>> memcpy() instead of the cumbersome __may_alias__ construct to
> >> retrieve
> >>>> the data.
> >>>>
> >>>
> >>> Here:
> >>> https://www.agner.org/optimize/instruction_tables.pdf
> >>> it lists the latency of vmovdqa (aligned) as 6 cycles and the
> latency
> >> for
> >>> vmovdqu (unaligned) as 7 cycles. So I guess there can be some
> >> difference.
> >>> Although in practice I'm not sure what difference it makes. I've
> not
> >> seen any
> >>> difference in runtime between the two versions.
> >>>
> >>
> >> Correction to my comment:
> >> Those stats are for some older CPU. For some newer CPUs such as
> Tiger
> >> Lake the stats seem to be the same regardless of aligned or
> unaligned.
> >>
> >
> > I agree that the memcpy method is more elegant and easy to read.
> >
> > However, we would need to performance test the modified checksum
> function with a large number of CPUs to prove that we don't introduce a
> performance regression on any CPU architecture still supported by DPDK.
> And Emil already found a CPU where it costs 1 extra cycle per 16 bytes,
> which adds up to a total of ca. 91 extra cycles on a 1460 byte TCP
> packet.
> >
> 
> I think you've misunderstood what latency means in such tables. It's a
> data dependency thing, not a measure of throughput. The throughput is
> *much* higher. My guess would be two such instruction per clock.
> 
> For your 1460 bytes example, my Zen3 AMD needs performs identical with
> both the current DPDK implementation, your patch, and a memcpy()-ified
> version of the current implementation. They all need ~130 clock
> cycles/packet, with warm caches. IPC is 3 instructions per cycle, but
> obvious not all instructions are SIMD.

You're right, I wasn't thinking deeper about it before extrapolating.

Great to see some real numbers! I wish someone would do the same testing on an old ARM CPU, so we could also see the other end of the scale.

> The main issue with checksumming on the CPU is, in my experience, not
> that you don't have enough compute, but that you trash the caches.

Agree. I have noticed that x86 has "non-temporal" instruction variants to load/store data without trashing the cache entirely.

A variant of the checksum function using such instructions might be handy.

Variants of the memcpy function using such instructions might also be handy for some purposes, e.g. copying the contents of packets, where the original and/or copy will not accessed shortly thereafter.

> > So I opted for a solution with zero changes to the inner loop, so no
> performance retesting is required (for the previously supported use
> cases, where the buffer is aligned).
> >
> 
> You will see performance degradation with this solution as well, under
> certain conditions. For unaligned 100 bytes of data, the current DPDK
> implementation and the memcpy()-fied version needs ~21 cc/packet. Your
> patch needs 54 cc/packet.

Yes, it's a tradeoff. I exclusively aimed at maintaining performance for the case with aligned buffers (under all circumstances, with all CPUs etc.), and ignored how it affects the performance for the case with unaligned buffers.

Unlike this patch, the memcpy() variant has no additional branches for the unaligned case, so its performance should be generally unaffected by the buffer being aligned or not. However, I don't have sufficient in-depth CPU knowledge to say if this also applies to RISCV and older ARM CPUs still supported by DPDK.

I don't have strong feelings for this patch, so you can provide a memcpy() based alternative patch, and we will let the community decide which one they prefer.

> But the old version didn't support unaligned accesses? In many compiler
> flag/machine combinations it did.

It crashed in some cases. That was the point of the bug report [1], and the reason to provide a patch.

[1] https://bugs.dpdk.org/show_bug.cgi?id=1035