Hi,
My test case is just a simple 1D heat equation solved using explicit finite difference method. So, while with block distribution there shouldn't be an excessive amount of communication, the program exhibits fine grained parallelism. According to my quick calculations, the total amount of data transferred between locales during the program run(using two locales) should less than 50 kilobytes, excluding all chapel's internal communications.
This also happens with the standard block distribution.
I tried remote caching but it didn't improve things. Also I can't rule out the possibility of there being something (very) wrong with the LAN, however since this also happens on one machine I didn't just yet investigate this further.
19.02.2015, 17:45, "Greg Titus" <[email protected]>:
Hi John --
(You already know the following as general knowledge, but I thought I'd include it for people newer to multi-locale programming who may be following this conversation.)
For context, with current Chapel it's normal for programs to suffer some performance loss when moving from single-locale to multi-locale execution. Using multiple locales offers more opportunity for parallelism, but at the cost of reduced intra-task performance due to network communication required for inter-locale variable references. The effect varies across programs depending on how much remote communication they do, but since a remote reference can easily take 1000 times as long as a local one, it doesn't take much to have a big effect. For example, with Chapel 1.10 on a Cray XC we don't see any drop-off in performance from 1 locale to 2 on the Stream benchmark, but our Stream doesn't do many inter-locale memory references. But for the RA benchmark, which does a lot of inter-locale references (in fact that's what it's measuring), our multi-locale performance doesn't match that on a single locale until we get up to 8-32 locales, depending on circumstances. And the Cray XC has a very high-performance network compared to UDP or MPI over ethernet.
That said, the >100x slowdown you're seeing seems a little high, unless your test case is really doing a lot of remote references. If it isn't, or at least shouldn't be, perhaps you're seeing a lot of remote communication for internal references to meta-data, within your distribution code? If this is the case, then turning on remote caching could well improve matters. In fact that might be a good test to rule this hypothesis in or out.
A secondary effect with GASNET_SPAWNFN=L could be oversubscription of the processor cores due to running more than one Chapel locale per compute node. To reduce the level of oversubscription you could set CHPL_NUM_THREADS_PER_LOCALE to the number of compute-node cores divided by the number of locales you're running on the compute node, but don't set it to less than 2 or you could deadlock/livelock due to internal starvation. However, if you're seeing the same slowdown with GASNET_SPAWNFN=S and one Chapel locale per compute node then I don't think this is something that is afflicting you right now.
hope this helps,
greg
On 2/19/2015 4:42 AM, John MacFrenz wrote:Hi,I'll give --cache-remote a try later. However for now I'm facing some problems which definitely should be solved first...The problem I'm having is that using GASNET_SPAWNF=L with UDP-conduit with more than one locale causes program to run _very_ slowly. For example, with one locale my test program did take 0.20 sec to run. With two locales it took 65 seconds. Same can be observed when running with GASNET_SPAWNF=S with UDP conduit on two separate machines. Using MPI conduit didn't make difference. Here's the environment variables I'm usingCHPL_HOME: /home/share/chapel/chapel-git
script location: /home/share/chapel/chapel-git/util
CHPL_HOST_PLATFORM: linux32
CHPL_HOST_COMPILER: gnu
CHPL_TARGET_PLATFORM: linux32
CHPL_TARGET_COMPILER: gnu
CHPL_TARGET_ARCH: unknown
CHPL_LOCALE_MODEL: flat
CHPL_COMM: gasnet
CHPL_COMM_SUBSTRATE: udp
CHPL_GASNET_SEGMENT: everything
CHPL_TASKS: fifo
CHPL_LAUNCHER: amudprun
CHPL_TIMERS: generic
CHPL_MEM: cstdlib
CHPL_MAKE: gmake
CHPL_ATOMICS: intrinsics
CHPL_NETWORK_ATOMICS: none
CHPL_GMP: none
CHPL_HWLOC: none
CHPL_REGEXP: none
CHPL_WIDE_POINTERS: struct
CHPL_LLVM: none
CHPL_AUX_FILESYS: noneAny idea what could be causing this? As I said on some previous post my target environment is heterogeneous (all x86, though) commodity cluster with ethernet connections, so either UDP or MPI conduit is one I'd use..18.02.2015, 23:41, "Greg Titus" <[email protected]>:Hi John --
A little bit of follow-up to what Michael says here ...
The "nemesis" he refers to is the internal name of the particular Qthreads scheduler we use when CHPL_LOCALE_MODEL=flat. Our understanding is that the nemesis scheduler currently doesn't move qthreads (and by extension, Chapel tasks) from pthread to pthread, which would break the use of pthread local storage inside the remote caching implementation. But there are significant caveats here:Taken together, this basically says that although we haven't observed remote caching failures with qthreads, that shouldn't be taken as evidence that it definitely does work now or will work in the future.
- We use a different Qthreads scheduler when CHPL_LOCALE_MODEL=numa, and that one definitely does move qthreads (thus Chapel tasks) from pthread to pthread.
- We can't guarantee that we'll always use "nemesis" with the flat locale model.
- We can't guarantee that, even if we do keep using it, "nemesis" will continue to not move qthreads (thus Chapel tasks) from pthread to pthread.
greg
On 2/18/2015 2:31 PM, Michael Ferguson wrote:Hi -
One more thing about the --cache-remote feature, just to be clear and for future reference:
The remote caching depends on pthread local storage, and Chapel task movement among worker pthreads in Qthreads-based tasking could break it. So far we haven't seen this happen, but we cannot guarantee it won't. Symptoms of a failure could include silent wrong answers or segfaults, either of which could be solid or intermittent/sporadic.
I *think* that this problem won't come up with the nemesis qthreads scheduler, but we need to do some careful analysis before we can declare the --cache-remote feature safe to use with qthreads.
Cheers,
-michael
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