[petsc-users] Understanding the memory bandwidth

[Matthew Knepley]

On Thu, Aug 13, 2015 at 3:22 PM, Justin Chang <jychang48 at gmail.com> wrote:

> On Thu, Aug 13, 2015 at 1:04 PM, Jed Brown <jed at jedbrown.org> wrote:
> > It looks like with one core/socket, all your memory sits over one
> > channel.  You can play tricks to avoid that or use 4 cores/socket in
> > order to use all memory channels.
>
> How do I play these tricks?
>
> > So this is a pretty low fraction (55%) of 59.7*2 = 119.4.  I suspect
> > your memory or motherboard is at most 1600 MHz, so your peak would be
> > 102.4 GB/s.
>
> > You can check this as root using "dmidecode --type 17", which should
> > give one entry per channel, looking something like this:
> >
> > Handle 0x002B, DMI type 17, 34 bytes
> > Memory Device
> >         Array Handle: 0x002A
> >         Error Information Handle: 0x002F
> >         Total Width: Unknown
> >         Data Width: Unknown
> >         Size: 4096 MB
> >         Form Factor: DIMM
> >         Set: None
> >         Locator: DIMM0
> >         Bank Locator: BANK 0
> >         Type: <OUT OF SPEC>
> >         Type Detail: None
> >         Speed: Unknown
> >         Manufacturer: Not Specified
> >         Serial Number: Not Specified
> >         Asset Tag: Unknown
> >         Part Number: Not Specified
> >         Rank: Unknown
> >         Configured Clock Speed: 1600 MHz
>
> I have no root access. Is there another way to confirm the clock speed?
>
> ---
>
> So if I have two sockets per node, then the theoretical peak bandwidth
> is actually double than what I thought (whether it be 119.4 GB/s or
> 102.4 GB/s). And if 8 cores really is the optimal number to use for a
> single compute node, why are there 20 totals to begin with? Or would
> this depend on the particular application?
>

Kind Answer: Different application have different needs

Cynical Answer: Computer companies sell you what they can produce,
lots of cores, not what you need, lots of bandwidth. Bandwidth is very
expensive and there are technical limits.


> Also, can someone elaborate on the difference between the words
> "core", "processor", and "thread"?
>

A core and a processor are hardware terms. I think they are both fuzzy,
but I understand a core to be something that can carry a thread of
execution,
namely a program counter, instruction and data stream, and compute
something.
A thread is a logical construct for talking about an execution stream.

   Matt

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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