[mpich-discuss] Clock synchronization

[Ravi Prakash]

Hi Jim and Pavan,

Thank you for your replies. I meant the time I'd get by calling MPI_Wtime() .Section 7.1.1.4 (Clock Synchronization) on http://www.mpi-forum.org/docs/mpi-11-html/node143.html states "the expectation is that the variation in time, as measured by calls  
to  MPI_WTIME, will be less then one half the round-trip  
time for an  MPI message of length zero."


So I guess every time I call MPI_Init(), the clocks (per process) are synchronized. My original question was what algorithm is used to sync that time across multiple machines. However if the variation is a few 10s of ms I should probably be fine. I'd been in trouble if it was 100s of ms. I've got only 3-4 machines, so hopefully it shouldn't be too bad.


Thanks,
Ravi


[please keep mpich-discuss cc'ed].

There is no real communication during synchronization. Zero-byte 
messages are exchanged. The algorithm used is logarithmic with respect 
to the number of processes.

  -- Pavan

On 04/02/2011 02:52 AM, Ravi Prakash wrote:
> Thanks Pavan! :) Do you know if synchronization is done by splitting the
> communication time in half or somesuch algorithm?
>
> Cheers
> Ravi
>

________________________________
From: James Dinan <dinan at mcs.anl.gov>
To: mpich-discuss at mcs.anl.gov
Sent: Saturday, April 2, 2011 8:56 AM
Subject: Re: [mpich-discuss] Clock synchronization

Hi Ravi?

Do you mean synchronization of the time of day on the machines?  If so, 
MPI doesn't synchronize the clock on each machine.  If you want to do 
this, you need a network time daemon or whatever equivalent there is for 
windows.  This is outside of the scope of MPI, but someone else may be 
able to suggest what you need.

Best,
  ~Jim.

On 4/1/11 11:28 PM, Ravi Prakash wrote:
> Hi,
>
> I'm a n00b and I'm wondering if there is any documentation on what clock
> synchronization algorithm MPICH2 uses. I need to trigger events at the
> same time across multiple machines. What accuracy am I looking at?
>
> My machines are Windows Vista boxes with Dual dual-core Xeons connected
> by an ethernet 100Mbps LAN. Please let me know if there are any more
> relevant factors needed for an approximate figure. Is it a few ms / 10's
> of ms / 100's of ms / god forbid seconds? :)
>
> Cheers
> Ravi
>
>
>
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> mpich-discuss at mcs.anl.gov
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