Subject: Re: [RFC,net-next,x86 0/6] Nontemporal copies in unix socket write path

From: Joe Damato <[email-masked]>
Date: Thu, 12 May 2022 15:53:05 -0700

On Thu, May 12, 2022 at 12:46:08PM -0700, Jakub Kicinski wrote:
> On Wed, 11 May 2022 18:01:54 -0700 Joe Damato wrote:
> > > Is there a practical use case?  
> > 
> > Yes; for us there seems to be - especially with AMD Zen2. I'll try to
> > describe such a setup and my synthetic HTTP benchmark results.
> > 
> > Imagine a program, call it storageD, which is responsible for storing and
> > retrieving data from a data store. Other programs can request data from
> > storageD via communicating with it on a Unix socket.
> > 
> > One such program that could request data via the Unix socket is an HTTP
> > daemon. For some client connections that the HTTP daemon receives, the
> > daemon may determine that responses can be sent in plain text.
> > 
> > In this case, the HTTP daemon can use splice to move data from the unix
> > socket connection with storageD directly to the client TCP socket via a
> > pipe. splice saves CPU cycles and avoids incurring any memory access
> > latency since the data itself is not accessed.
> > 
> > Because we'll use splice (instead of accessing the data and potentially
> > affecting the CPU cache) it is advantageous for storageD to use NT copies
> > when it writes to the Unix socket to avoid evicting hot data from the CPU
> > cache. After all, once the data is copied into the kernel on the unix
> > socket write path, it won't be touched again; only spliced.
> > 
> > In my synthetic HTTP benchmarks for this setup, we've been able to increase
> > network throughput of the the HTTP daemon by roughly 30% while reducing
> > the system time of storageD. We're still collecting data on production
> > workloads.
> > 
> > The motivation, IMHO, is very similar to the motivation for
> > NETIF_F_NOCACHE_COPY, as far I understand.
> > 
> > In some cases, when an application writes to a network socket the data
> > written to the socket won't be accessed again once it is copied into the
> > kernel. In these cases, NETIF_F_NOCACHE_COPY can improve performance and
> > helps to preserve the CPU cache and avoid evicting hot data.
> > 
> > We get a sizable benefit from this option, too, in situations where we
> > can't use splice and have to call write to transmit data to client
> > connections. We want to get the same benefit of NETIF_F_NOCACHE_COPY, but
> > when writing to Unix sockets as well.
> > 
> > Let me know if that makes it more clear.
> 
> Makes sense, thanks for the explainer.
> 
> > > The patches look like a lot of extra indirect calls.  
> > 
> > Yup. As I mentioned in the cover letter this was mostly a PoC that seems to
> > work and increases network throughput in a real world scenario.
> > 
> > If this general line of thinking (NT copies on write to a Unix socket) is
> > acceptable, I'm happy to refactor the code however you (and others) would
> > like to get it to an acceptable state.
> 
> My only concern is that in post-spectre world the indirect calls are
> going to be more expensive than an branch would be. But I'm not really
> a mirco-optimization expert :)

Makes sense; neither am I, FWIW :)

For whatever reason, on AMD Zen2 it seems that using non-temporal
instructions when copying data sizes above the L2 size is a huge
performance win (compared to the kernel's normal temporal copy code) even
if that size fits in L3.

This is why both NETIF_F_NOCACHE_COPY and MSG_NTCOPY from this series seem
to have such a large, measurable impact in the contrived benchmark I
included in the cover letter and also in synthetic HTTP workloads.

I'll plan on including numbers from the benchmark program on a few other
CPUs I have access to in the cover letter for any follow-up RFCs or
revisions.

As a data point, there has been similar-ish work done in glibc [1] to
determine when non-temporal copies should be used on Zen2 based on the size
of the copy. I'm certainly not a micro-arch expert by any stretch, but the
glibc work plus the benchmark results I've measured seem to suggest that
NT-copies can be very helpful on Zen2.

Two questions for you:

 1. Do you have any strong opinions on the sendmsg flag vs a socket option?

 2. If I can think of a way to avoid the indirect calls, do you think this
    series is ready for a v1? I'm not sure if there's anything major that
    needs to be addressed aside from the indirect calls.

I'll include some documentation and cosmetic cleanup in the v1, as well.

Thanks,
Joe

[1]: https://sourceware.org/pipermail/libc-alpha/2020-October/118895.html