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From: Rick Jones on 13 Jul 2010 18:45
Rahul <nospam(a)nospam.invalid> wrote:
> Rick Jones <rick.jones2(a)hp.com> wrote in
> news:i1ia16$7sa$2(a)usenet01.boi.hp.com:

> > Is a LAN RTT even a non-trivial fraction of the service time of your
> > application?

> Yes, I think it is. It is a MPI application (computational chemistry :
> VASP) using distributed memory that does a fair amount of small-packet
> traffic.

I thought the goal of most MPI applications was to minimize the number
of MPI message passings?

> > Just *how* latency sensitive?

> It is hard to say since I don't know of a way to vary latency on demand (is
> there a way? I'd be eager to know!) to test response. These are the data
> points I have:

> Using 6 servers with 8 cores each.
> RT Latency Job Runtime (normalised secs)
> 130 usec 10x
> 18 usec 1.5x
> 7 usec 1x

> 18 usec is my current network.

If your cluster is limited to 6 nodes, you might want to consider a
"cluster in a box" with an 8S system, you should get rather better
than 18 usec RTT over loopback.

rick jones

> >If one request/response pair out of N,
> > where N could be quite large depending on how fast you are running,
> > has an extra RTT added to it is that really going to cause problems?

> You are probably right. It won't matter. It depends on how large is N.

> --
> Rahul

--
The glass is neither half-empty nor half-full. The glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
From: Rick Jones on 13 Jul 2010 18:49
Moe Trin <ibuprofin(a)painkiller.example.tld.invalid> wrote:
> They're not really ARP caches, so much as a lookup table of which
> host is connected to which hose. When the switch looses it - or when
> it can't figure out which port a host is on, it will often broadcast
> the packet to all ports - not good for efficiency.

Given the added meaning of "broadcast" it might not be a bad idea to
put that as "it will transmit the packet on all ports" :)

rick jones
--
Wisdom Teeth are impacted, people are affected by the effects of events.
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
From: Moe Trin on 14 Jul 2010 16:15
On Tue, 13 Jul 2010, in the Usenet newsgroup comp.os.linux.networking, in
article <i1iqhf$ds8$2(a)usenet01.boi.hp.com>, Rick Jones wrote:

>Moe Trin <ibuprofin(a)painkiller.example.tld.invalid> wrote:

>> When the switch looses it - or when it can't figure out which port
>> a host is on, it will often broadcast the packet to all ports - not
>> good for efficiency.

>Given the added meaning of "broadcast" it might not be a bad idea to
>put that as "it will transmit the packet on all ports" :)

Semantics - but yeah, I'd agree with that.

Old guy
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