Opteron errata / nVidia issues - links and stuff [was Re: [Pc_Support] It's here!]

[Bryan J. Smith]

On Sat, 2005-11-05 at 13:42 -0500, Austin Denyer wrote:
> The new Intel dual-cores are downright pathetic from the reviews I've
> seen so far.

Until Intel allows processors to access memory and I/O independently of
each other, they will have poor performance.  At the same time, because
Intel has never allowed such, they are going to find they are 5 years
behind AMD when they do next year.

> Here's a link to the kernel patch issue.  It's long, so I've
> reproduced the clip below.
> http://www.kernel.org/pub/linux/kernel/v2.6/testing/ChangeLog-2.6.14-rc2

Yep, found that in the AMD Errata -- Rev. Guide 3.07, page 16 (fall
2003):  
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25759.PDF

Coherency issues with a hardware-side hack that AMD came up.  Long story
short, 32-bit NT kernels use PAE, and that is a major performance hit.
So AMD came up with a way to filter out required TLB flushes.  I'm sure
this gives all sorts of performance bonuses.

The downside is ... as always ... AMD CPUs accessing things
independently -- whether via the EV6 crossbar switch in Athlon MP, or
now the NUMA/HyperTransport multi-point in Opteron 2xx/8xx.  So YACI
(yet another coherency issue) with AMD's design in MP modes (where CPU
states will differ), although they've come a long way from Athlon MP.

The workaround is to not use the hardware-side hack.  This patch merely
disables it to prevent its use.

Although AMD is planning a new version of the hack that solves the
coherency issue (already implemented in newer Rev. E???), it can be
solved with a kernel that doesn't rely on old assumptions of all CPUs
having coherent TLBs because they all use the same, shared interconnect
(i.e., old Intel).  That seems to be what Linus wants, which will only
help Linux's scalability on NUMA/multi-point architectures.

Coherency issues are always the problem when you don't use a single
"memory controller hub" point for all system communications.


-- 
Bryan J. Smith     b.j.smith at ieee.org     http://thebs413.blogspot.com
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