Date: Fri, 22 Jul 2016 09:54:08 +0800 From: Sepherosa Ziehau <sepherosa@gmail.com> To: Adrian Chadd <adrian@freebsd.org> Cc: Andrew Gallatin <gallatin@cs.duke.edu>, FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: proposal: splitting NIC RSS up from stack RSS Message-ID: <CAMOc5cxEWqWOMPSXFe3=N5S93bs8RO-XX22QghtHd8vC5xuNjA@mail.gmail.com> In-Reply-To: <CAJ-VmomHYVCknVkDLF%2Bb8Gc5wBWxkddEMY3dhvbxJihLZHyTLg@mail.gmail.com> References: <CAJ-Vmo=Wj3ZuC6mnVCxonQ74nfEmH7CE=TP3xhLzWifdBxxfBQ@mail.gmail.com> <306af514-70ff-f3bf-5b4f-da7ac1ec6580@cs.duke.edu> <CAJ-VmomHYVCknVkDLF%2Bb8Gc5wBWxkddEMY3dhvbxJihLZHyTLg@mail.gmail.com>
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On Fri, Jul 22, 2016 at 6:39 AM, Adrian Chadd <adrian@freebsd.org> wrote:
> hi,
>
> Cool! Yeah, the RSS bits thing can be removed, as it's just doing a
> bitmask instead of a % operator to do mapping. I think we can just go
> to % and if people need the extra speed from a power-of-two operation,
> they can reintroduce it.
I thought about it a while ago (the most popular E5-2560v{1,2,3} only
has 6 cores, but E5-2560v4 has 8 cores! :). Since the raw RSS hash
value is '& 0x1f' (I believe most of the NICs use 128 entry indirect
table as defined by MS RSS) to select an entry in the indirect table,
simply '%' on the raw RSS hash value probably will not work properly;
you will need (hash&0x1f)%mp_ncpus at least. And well, since the
indirect table's size if 128, you still will get some uneven CPU
workload for non-power-of-2 cpus. And if you take cpu affinity into
consideration, the situation will be even more complex ...
Thanks,
sephe
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