All of this is completely expected. Due to the requirements of the standard it 
is difficult to make use of network atomics even for MPI_Compare_and_swap 
(MPI_Accumulate and MPI_Get_accumulate spoil the party). If you want 
MPI_Fetch_and_op to be fast set this MCA parameter:


Shared lock is slower than an exclusive lock because there is an extra lock 
step as part of the accumulate (it isn't needed if there is an exclusive lock). 
When setting the above parameter you are telling the implementation that you 
will only be using a single count and we can optimize that with the hardware. 
The RMA working group is working on an info key that will essentially do the 
same thing.

Note the above parameter won't help you with IB if you are using UCX unless you 
set this (master only right now):




Though there may be a way to get osc/ucx to enable the same sort of 
optimization. I don't know.


On Nov 06, 2018, at 09:38 AM, Joseph Schuchart <> wrote:


I am currently experimenting with MPI atomic operations and wanted to
share some interesting results I am observing. The numbers below are
measurements from both an IB-based cluster and our Cray XC40. The
benchmarks look like the following snippet:

if (rank == 1) {
uint64_t res, val;
for (size_t i = 0; i < NUM_REPS; ++i) {
MPI_Fetch_and_op(&val, &res, MPI_UINT32_T, 0, 0, MPI_SUM, win);
MPI_Win_flush(target, win);

Only rank 1 performs atomic operations, rank 0 waits in a barrier (I
have tried to confirm that the operations are done in hardware by
letting rank 0 sleep for a while and ensuring that communication
progresses). Of particular interest for my use-case is fetch_op but I am
including other operations here nevertheless:

* Linux Cluster, IB QDR *
average of 100000 iterations

Exclusive lock, MPI_UINT32_T:
fetch_op: 4.323384us
compare_exchange: 2.035905us
accumulate: 4.326358us
get_accumulate: 4.334831us

Exclusive lock, MPI_UINT64_T:
fetch_op: 2.438080us
compare_exchange: 2.398836us
accumulate: 2.435378us
get_accumulate: 2.448347us

Shared lock, MPI_UINT32_T:
fetch_op: 6.819977us
compare_exchange: 4.551417us
accumulate: 6.807766us
get_accumulate: 6.817602us

Shared lock, MPI_UINT64_T:
fetch_op: 4.954860us
compare_exchange: 2.399373us
accumulate: 4.965702us
get_accumulate: 4.977876us

There are two interesting observations:
a) operations on 64bit operands generally seem to have lower latencies
than operations on 32bit
b) Using an exclusive lock leads to lower latencies

Overall, there is a factor of almost 3 between SharedLock+uint32_t and
ExclusiveLock+uint64_t for fetch_and_op, accumulate, and get_accumulate
(compare_exchange seems to be somewhat of an outlier).

* Cray XC40, Aries *
average of 100000 iterations

Exclusive lock, MPI_UINT32_T:
fetch_op: 2.011794us
compare_exchange: 1.740825us
accumulate: 1.795500us
get_accumulate: 1.985409us

Exclusive lock, MPI_UINT64_T:
fetch_op: 2.017172us
compare_exchange: 1.846202us
accumulate: 1.812578us
get_accumulate: 2.005541us

Shared lock, MPI_UINT32_T:
fetch_op: 5.380455us
compare_exchange: 5.164458us
accumulate: 5.230184us
get_accumulate: 5.399722us

Shared lock, MPI_UINT64_T:
fetch_op: 5.415230us
compare_exchange: 1.855840us
accumulate: 5.212632us
get_accumulate: 5.396110us

The difference between exclusive and shared lock is about the same as
with IB and the latencies for 32bit vs 64bit are roughly the same
(except for compare_exchange, it seems).

So my question is: is this to be expected? Is the higher latency when
using a shared lock caused by an internal lock being acquired because
the hardware operations are not actually atomic?

I'd be grateful for any insight on this.


Dipl.-Inf. Joseph Schuchart
High Performance Computing Center Stuttgart (HLRS)
Nobelstr. 19
D-70569 Stuttgart

Tel.: +49(0)711-68565890
Fax: +49(0)711-6856832
users mailing list
users mailing list

Reply via email to