netdev - Re: Receive side performance issue with multi-10-GigE and NUMA

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Message-Id: <1250774056.16001.157.camel@localhost>
Date:	Thu, 20 Aug 2009 14:14:16 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Bill Fink <billfink@...dspring.com>
Cc:	Aviv Greenberg <avivgnet@...il.com>,
	Andrew Gallatin <gallatin@...i.com>, netdev@...r.kernel.org
Subject: Re: Receive side performance issue with multi-10-GigE and NUMA

On Thu, 2009-08-20 at 03:26 -0400, Bill Fink wrote:
> On Sat, 15 Aug 2009, Aviv Greenberg wrote:
> 
> > >  There may be something in the chipset
> > 
> > shooting in the dark: when you lspci -vvv and check the MaxPayload and
> > MaxReadReq values for the myri devices - what are the values and are
> > they equal? Are they the same on all your platforms?
> 
> IIRC, under DevCap they indicated MaxPayload 4096 bytes, and under
> DevCtl they indicated MaxPayload 128 bytes and MaxReadReq 4096 bytes,
> and was the same on both the Asus and SuperMicro systems.  I will
> doublecheck tomorrow at work.  I am not clear on the meanings of
> the different parameters.  And is DevCtl for PCI control messages
> and DevCap for actual data transfers or something else?

DevCap is the capability register, which is read-only; DevCtl is the
control register which holds the actual settings.

MaxPayload is the MTU and MRU for PCIe packets.  Each sub-tree of
devices connected to a single PCIe root port needs to have MaxPayload
set consistently.  MaxReadReq is the maximum size of any DMA read
request.  It is a per-device setting (or possibly per-function; I
forget).  It can be much larger than MaxPayload since read completions
can be fragmented.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
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