Massively parallel on single core? Post: 302394358

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Special Forums UNIX and Linux Applications High Performance Computing Massively parallel on single core? Post 302394358 by Andre_Merzky on Thursday 11th of February 2010 07:48:20 AM

02-11-2010

Registered User

Massively parallel on single core?

Hia all,

I am not sure how many people actually follow the HPC forum on unix.com, but you may be interested in discussing the following (academic) problem:

Assume you want to run a *very* large number (say 100.000) of very lightweight synchronous operations. As an example, assume that you want to run 100.000 instances of

Code:

sleep (3600); // thats one hour sleep

The trivial (aka braindead) approach would be

Code:

for ( int i = 0; i < 100000; i++ )
{
  ::sleep (3600);
}

Takes about 15 years to finish ;-)

One could start 1000 threads, and run a sleep in each of them. That reduces the runtime to 100 hours - still 4 days, and the system is totally idle all the time.

So, using more threads? Won't work, as the max-threads-per-process limit will be hit at some point.

So, spawn 100 processes which spawn 1000 threads each?
The max-threads-per-process limit is, on Linux, close to the max-threads-per-system limit, so that won't work. On other Unixes that is different, but I don't think you get 100.000 threads on a normal single CPU system. Do you?

So, what would your approach be?

I am not looking for a sleep replacement: so saying that I should set alarm or something similar is of not much use. Sleep is obviously only an example here - replace it with an extremely lightweight job, like running a very time consuming synchronous remote operation.

I am looking forward to the ideas you guys can come up with! :-)

Cheers, Andre.

Last edited by Andre_Merzky; 02-11-2010 at 08:48 AM.. Reason: layout...

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LEARN ABOUT OSF1

nfsd

nfsd(8) 						      System Manager's Manual							   nfsd(8)

NAME

       nfsd - The remote NFS compatible server

SYNOPSIS

       /usr/sbin/nfsd [-t num_tcpthreads] [-u num_udpthreads]

       The following form of the nfsd command is not recommended and is supported only for backward compatibility:

       /usr/sbin/nfsd [numthreads]

FLAGS

       Specifies  a  number  of  TCP server threads (per RAD) to spawn.  A value of 8 is recommended as a start.  Specifies a number of UDP server
       threads (per RAD) to spawn.  A value of 8 is recommended as a start.

DESCRIPTION

       The nfsd daemon runs on a server machine to service NFS requests from client machines.  The daemon spawns a number of server  threads  that
       process NFS requests from client machines.  At least one server thread must be running for a machine to operate as a server.

       There  are  two	types of server threads: a server thread that processes NFS requests sent using TCP and a server thread that processes NFS
       requests sent using UDP.  This is necessary because the kernel paths for UDP and TCP NFS messages are different.  The -t  option  specifies
       the number of TCP threads to run and the -u option specifies the number of UDP threads to run.

       On  systems that support Cache Coherent NUMA, the number of threads is per Resource Affinity Domain (RAD).  As you add RADs, the NFS server
       will automatically scale by creating additional threads.  NFS requests are processed by a particular RAD based on the file being  accessed;
       this  confines cached information about a file to a single RAD for efficiency. See numa_intro(3) for more information on the NUMA architec-
       ture.

       If you use the SysMan Menu to configure NFS, it sets the default at 8 UDP and 8 TCP threads.  However, a user can have any  number  of  TCP
       and  UDP  nfsd  threads running up to a maximum of 128 threads.	The optimal number of TCP server threads and UDP server threads depends on
       many factors.  See nfsiod(8) for more information.

       The server threads are implemented as kernel threads; they are part of Process ID 0, not the nfsd process.  The ps  axml  command  displays
       idle server threads under PID 0.  Idle threads will be waiting on nfs_udp_wait or nfs_tcp_wait. Therefore, if 16 server threads are config-
       ured, only one nfsd process is displayed in the output from the ps command,  although  16  server  threads  are	available  to  handle  NFS
       requests.

       Files  that  are larger than 2 gigabytes are exported as 2 gigabyte files when accessed by NFS Version 2.  NFS Version 2 is a 32-bit proto-
       col, therefore, the size and offset fields are 32-bit quantities (on Alpha UFS they are 64-bit  quantities).  Use  caution  when  accessing
       files larger than 2 gigabytes from NFS clients.

EXAMPLES

       In the following example, 16 threads are run (8 for TCP and 8 for UDP): nfsd -t 8 -u 8

FILES

       Specifies  the  command path Specifies the file for logging startup errors (before the server threads are started).  Specifies the file for
       logging NFS errors (after the server threads are started).

RELATED INFORMATION

       Commands: mount(8), mountd(8), nfsconfig(8), nfsstat(8), portmap(8)

       System calls: nfssvc(2) delim off

																	   nfsd(8)