[mpich-discuss] Clock synchronization

[Pavan Balaji]

Ravi,

No, the clocks are not synchronized every time you call MPI_Wtime(). 
Just the local time is returned.

The part you are reading is only when the attribute MPI_WTIME_IS_GLOBAL 
is set.

  -- Pavan

On 04/02/2011 10:42 PM, Ravi Prakash wrote:
> 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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-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji