Re: [mpich-discuss] osu_latency test: why 8KB takes less time than 4KB and 2KB takes less time than 1KB?

2 Jul 2018

      Hi Abu,

Thanks for reporting this. Can you please try the latest release with 
ch3/sock and see if you still have this error ?

Min
On 2018/07/01 21:47, Abu Naser wrote:
...
Hello Min,
After compiling my mpich-3.2.1 with sock, while I was trying to runï¿½ 
any program including osuï¿½benchmark orï¿½examples/cpi in two machines, I 
have received following error -
/Process 3 of 4 is on dhcp16194/
/Process 1 of 4 is on dhcp16194/
/Process 0 of 4 is on dhcp16198/
/Process 2 of 4 is on dhcp16198/
/Fatal error in PMPI_Bcast: Unknown error class, error stack:/
/PMPI_Bcast(1600)............................: 
MPI_Bcast(buf=0x7ffc1808542c, count=1, MPI_INT, root=0, 
MPI_COMM_WORLD) failed/
/MPIR_Bcast_impl(1452).......................: /
/MPIR_Bcast(1476)............................: /
/MPIR_Bcast_intra(1249)......................: /
/MPIR_SMP_Bcast(1081)........................: /
/MPIR_Bcast_binomial(285)....................: /
/MPIC_Send(303)..............................: /
/MPIC_Wait(226)..............................: /
/MPIDI_CH3i_Progress_wait(242)...............: an error occurred while 
handling an event returned by MPIDU_Sock_Wait()/
/MPIDI_CH3I_Progress_handle_sock_event(698)..: /
/MPIDI_CH3_Sockconn_handle_connect_event(597): [ch3:sock] failed to 
connnect to remote process/
/MPIDU_Socki_handle_connect(808).............: connection failure 
(set=0,sock=1,errno=111:Connection refused)/
/MPIR_SMP_Bcast(1088)........................: /
/MPIR_Bcast_binomial(310)....................: Failure during collective/
/Fatal error in PMPI_Bcast: Other MPI error, error stack:/
/PMPI_Bcast(1600)........: MPI_Bcast(buf=0x7ffd9eeebdac, count=1, 
MPI_INT, root=0, MPI_COMM_WORLD) failed/
/MPIR_Bcast_impl(1452)...: /
/MPIR_Bcast(1476)........: /
/MPIR_Bcast_intra(1249)..: /
/MPIR_SMP_Bcast(1088)....: /
/MPIR_Bcast_binomial(310): Failure during collective/
I checked the mpichï¿½FAQ and also mpich discussion list. Based on that 
I have checked followingsand foundï¿½ï¿½theyï¿½are fine in my machines -
- firewall is disabled in both machine
- I can do password lessï¿½ssh in both machine
- /etc/hosts in both machine configured with ipï¿½address and name properly
- I have updated the library path and used absolute path for mpiexec
- Most importantly when I configured and build mpich with tcp, it 
works fine.
ï¿½I think I am missing something but could not figure out yet. Any help 
would be appreciated.
Thank you.
Best Regards,
Abu Naser
------------------------------------------------------------------------
*From:* Min Si <[email protected]>
*Sent:* Tuesday, June 26, 2018 12:54:29 PM
*To:* [email protected]
*Subject:* Re: [mpich-discuss] osu_latency test: why 8KB takes less 
time than 4KB and 2KB takes less time than 1KB?
Hi Abu,
I think the results are stable enough. Perhaps you could also try the 
following tests, and see if similar trend exists:
- MPICH/socket (set `--with-device=ch3:sock` at configure)
- A socket-based pingpong test without MPI.
At this point, I could not think of any MPI-specific design for 2k/8k 
messages. My guess is that it is related to your network connection.
Min
On 2018/06/24 11:09, Abu Naser wrote:
...
Hello Min and Jeff,
Here is my experiment results.ï¿½Default number of iterations in 
osu_latency for 0B ï¿½ 8KB is 10,000. With that setting I had run the 
osu_latency 100 times and found standard deviation 33 for 8KB message 
size.
So later I have set the iteration to 50,000 and 100,000 for 1KB ï¿½ 
16KB message size. Then run osu_latency for 100 times for each 
setting and take the average and standard deviation.
*Msg Size in Bytes*
*Avg time in us (50K iterations)*
*Avg time in us (100k iterations)*
*Standard deviation (50K iterations)*
*Standard deviation (100K iterations)*
1k
85.10
84.9
0.55
0.45
2k
75.79
74.63
5.09
4.44
4k
273.80
274.71
4.18
2.45
8k
258.56
249.83
21.14
28
16k
281.31
281.02
3.22
4.10
The standard deviation of 8K message is so high and that implies it 
actually not producing any consistent latency time. Looks likeï¿½that's 
the reason for 8K is taking less time than 4K.
Meanwhile, 2K has standard deviation less than 5 but 1K message 
latency timing are more densely populated than 2K. So probably this 
is the explanation for 2K message less latency time.
Thank you for your suggestions.
Best Regards,
Abu Naser
------------------------------------------------------------------------
*From:* Abu Naser
*Sent:* Wednesday, June 20, 2018 1:48:53 PM
*To:* [email protected] <mailto:[email protected]>
*Subject:* Re: [mpich-discuss] osu_latency test: why 8KB takes less 
time than 4KB and 2KB takes less time than 1KB?
Hello Min,
Thanks for the clarification.ï¿½ I will do the experiment.
Thanks.
Best Regards,
Abu Naser
------------------------------------------------------------------------
*From:* Min Si <[email protected]> <mailto:[email protected]>
*Sent:* Wednesday, June 20, 2018 1:39:30 PM
*To:* [email protected] <mailto:[email protected]>
*Subject:* Re: [mpich-discuss] osu_latency test: why 8KB takes less 
time than 4KB and 2KB takes less time than 1KB?
Hi Abu,
I think Jeff means that you should run your experiment with more 
iterations in order to get a stable results.
- Increase the iteration of for loop in each execution (I think osu 
benchmark allows you to set it)
- Run the experiments 10 or 100 times, and take the average and 
standard deviation.
If you see a very small standard deviation (e.g., <=5%), then the 
trend is stable and you might not see such gaps.
Best regards,
Min
On 2018/06/20 12:14, Abu Naser wrote:
...
Hello Jeff,
Yes, I am using a switch and other machinesï¿½are also connected with 
that switch.
If I remove other machines and just use my two node with the switch, 
then will it improve the performance by 200 ~ 400 iterations?
Meanwhile I will give a try with a single dedicated cable.
Thank you.
Best Regards,
Abu Naser
------------------------------------------------------------------------
*From:* Jeff Hammond <[email protected]> 
<mailto:[email protected]>
*Sent:* Wednesday, June 20, 2018 12:52:06 PM
*To:* MPICH
*Subject:* Re: [mpich-discuss] osu_latency test: why 8KB takes less 
time than 4KB and 2KB takes less time than 1KB?
Is the ethernet connection a single dedicated cable between the two 
machines or are you running through a switch that handles other 
traffic?
My best guess is that this is noise and that you may be able to 
avoid it by running a very long time, e.g. 10000 iterations.
Jeff
On Wed, Jun 20, 2018 at 6:53 AM, Abu Naser <[email protected] 
<mailto:[email protected]>> wrote:
Good day to all,
I had run point to point osu_latency test in two nodes for 200
    times.ï¿½ Followings are the average time in microsecond for
    various size of the messages -
1KBï¿½ï¿½ ï¿½84.8514 us
    2KBï¿½ï¿½ ï¿½73.52535 us
    4KBï¿½ï¿½ ï¿½272.55275 us
    8KBï¿½ï¿½ ï¿½234.86385 us
    16KBï¿½ï¿½ ï¿½288.88 us
    32KBï¿½ï¿½ ï¿½523.3725 us
    64KBï¿½ï¿½ ï¿½910.4025 us
From the above looks like, 2KB message has less latency than 1
    KB and 8KB has less latency than 4KB.
I was looking for explanation of this behaviorï¿½ but did not get any.
1. MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZEis set to 128KB. So none of
        the above message size is using Rendezvous protocol. Is
        there any partition inside eager protocol (e.g. 0 - 512
        bytes, 1KB - 8KB, 16KB - 64KB)? If yes then what are the
        boundaries for them? Can I log them with debug-event-logging?
Setup I am using:
- two nodes has intel core i7, one with 16gb memory another one 8gb
- mpich 3.2.1, configured and build to use nemesis tcp
- 1gb Ethernet connection
- NFS is using for sharing
- osu_latency : uses MPI_Send and MPI_Recv
- MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZE= 131072 (128KB)
Can anyone help me on that? Thanks in advance.
Best Regards,
Abu Naser
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-- 
Jeff Hammond
[email protected] <mailto:[email protected]>
http://jeffhammond.github.io/
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Min Si

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