[Rd] Get Logical processor count correctly whether NUMA is enabled or disabled

[Srinivasan, Arunkumar]

Dear Tomas, thank you for looking into this. Here's the output:

# number of logical processors - what detectCores() should return
out <- system("wmic cpu get numberoflogicalprocessors", intern=TRUE) 
[1] "NumberOfLogicalProcessors  \r" "22                         \r" "22                         \r"
[4] "20                         \r" "22                         \r" "\r"                           
sum(as.numeric(gsub("([0-9]+).*", "\\1", grep("[0-9]+[ \t]*", out, value=TRUE)))) 
# [1] 86

[I've asked the IT team to understand why one of the values is 20 instead of 22].

# number of cores - what detectCores(FALSE) should return
out <- system("wmic cpu get numberofcores", intern=TRUE)
[1] "NumberOfCores  \r" "22             \r" "22             \r" "20             \r" "22             \r"
[6] "\r"
sum(as.numeric(gsub("([0-9]+).*", "\\1", grep("[0-9]+[ \t]*", out, value=TRUE))))
# [1] 86

[Currently hyperthreading is disabled. So this output being identical to the previous output makes sense].

system("wmic computersystem get numberofprocessors")
NumberOfProcessors  
4

In addition, I'd also bring to your attention this documentation: https://docs.microsoft.com/en-us/windows/desktop/ProcThread/processor-groups on processor groups which explain how one should go about running a process ro run on multiple groups (which seems to be different to NUMA). All this seems overly complicated to allow a process to use all cores by default TBH.

Here's a project on Github 'fio' where the issue of running a process on more than 1 processor group has come up -  https://github.com/axboe/fio/issues/527 and is addressed - https://github.com/axboe/fio/blob/c479640d6208236744f0562b1e79535eec290e2b/os/os-windows-7.h . I am not sure though if this is entirely relevant since we would be forking new processes in R instead of allowing a single process to use all cores. Apologies if this is utterly irrelevant.

Thank you,
Arun.

From: Tomas Kalibera <tomas.kalibera using gmail.com> 
Sent: 21 August 2018 11:50
To: Srinivasan, Arunkumar <Arunkumar.Srinivasan using uk.mlp.com>; r-devel using r-project.org
Subject: Re: [Rd] Get Logical processor count correctly whether NUMA is enabled or disabled

Dear Arun,

thank you for the report. I agree with the analysis, detectCores() will only report logical processors in the NUMA group in which R is running. I don't have a system to test on, could you please check these workarounds for me on your systems?

# number of logical processors - what detectCores() should return
out <- system("wmic cpu get numberoflogicalprocessors", intern=TRUE)
sum(as.numeric(gsub("([0-9]+).*", "\\1", grep("[0-9]+[ \t]*", out, value=TRUE))))

# number of cores - what detectCores(FALSE) should return
out <- system("wmic cpu get numberofcores", intern=TRUE)
sum(as.numeric(gsub("([0-9]+).*", "\\1", grep("[0-9]+[ \t]*", out, value=TRUE))))

# number of physical processors - as a sanity check

system("wmic computersystem get numberofprocessors")

Thanks,
Tomas

On 08/17/2018 05:11 PM, Srinivasan, Arunkumar wrote:
Dear R-devel list,

R's detectCores() function internally calls "ncpus" function to get the total number of logical processors. However, this doesnot seem to take NUMA into account on Windows machines.

On a machine having 48 processors (24 cores) in total and windows server 2012 installed, if NUMA is enabled and has 2 nodes (node 0 and node 1 each having 24 CPUs), then R's detectCores() only detects 24 instead of the total 48. If NUMA is disabled, detectCores() returns 48.

Similarly, on a machine with 88 cores (176 processors) and windows server 2012, detectCores() with NUMA disabled only returns the maximum value of 64. If NUMA is enabled with 4 nodes (44 processors each), then detectCores() will only return 44. This is particularly limiting since we cannot get to use all processors by enabling/disabling NUMA in this case.

We think this is because R's ncpus.c file uses "PSYSTEM_LOGICAL_PROCESSOR_INFORMATION" (https://msdn.microsoft.com/en-us/library/windows/desktop/ms683194(v=vs.85).aspx) instead of "PSYSTEM_LOGICAL_PROCESSOR_INFORMATION_EX" (https://msdn.microsoft.com/en-us/library/windows/desktop/dd405488(v=vs.85).aspx). Specifically, quoting from the first link:

"On systems with more than 64 logical processors, the GetLogicalProcessorInformation function retrieves logical processor information about processors in the https://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx to which the calling thread is currently assigned. Use the https://msdn.microsoft.com/en-us/library/windows/desktop/dd405488(v=vs.85).aspx function to retrieve information about processors in all processor groups on the system."

Therefore, it might be possible to get the right count of total processors even with NUMA enabled by using "GetLogicalProcessorInformationEX".  It'd be nice to know what you think.

Thank you very much,
Arun.

--
Arun Srinivasan
Analyst, Millennium Management LLC 
50 Berkeley Street | London, W1J 8HD


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