Nathan,
Over the last couple of weeks I made some more interesting observations
regarding the latencies of accumulate operations on both Aries and
InfiniBand systems:
1) There seems to be a significant difference between 64bit and 32bit
operations: on Aries, the average latency for compare-exchange on 64bit
values takes about 1.8us while on 32bit values it's at 3.9us, a factor
of >2x. On the IB cluster, all of fetch-and-op, compare-exchange, and
accumulate show a similar difference between 32 and 64bit. There are no
differences between 32bit and 64bit puts and gets on these systems.
2) On both systems, the latency for a single-value atomic load using
MPI_Fetch_and_op + MPI_NO_OP is 2x that of MPI_Fetch_and_op + MPI_SUM on
64bit values, roughly matching the latency of 32bit compare-exchange
operations.
All measurements were done using Open MPI 3.1.2 with
OMPI_MCA_osc_rdma_acc_single_intrinsic=true. Is that behavior expected
as well?
Thanks,
Joseph
On 11/6/18 6:13 PM, Nathan Hjelm via users wrote:
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:
osc_rdma_acc_single_intrinsic=true
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):
btl_uct_transports=dc_mlx5
btl=self,vader,uct
osc=^ucx
Though there may be a way to get osc/ucx to enable the same sort of
optimization. I don't know.
-Nathan
On Nov 06, 2018, at 09:38 AM, Joseph Schuchart <schuch...@hlrs.de> wrote:
All,
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);
}
}
MPI_Barrier(MPI_COMM_WORLD);
```
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.
Cheers,
Joseph
--
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
E-Mail: schuch...@hlrs.de <mailto:schuch...@hlrs.de>
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