Skip to content
Snippets Groups Projects
Commit 6f418009 authored by Peter W. Draper's avatar Peter W. Draper
Browse files

Add some markup

parent fb3c2ff3
Branches
Tags v1.1
No related merge requests found
SWIFTmpistepsim
===============
This package is a standalone part of [SWIFT](http://www.swiftsim.com) that
This project is a standalone part of [SWIFT](http://www.swiftsim.com) that
aims to roughly simulate the MPI interactions that taking a single step of a
SWIFT simulation makes.
SWIFT simulation makes. Making it possible to more easily see the performance
of MPI calls, and also investigations of tuning more obvious.
The actual process within SWIFT is that queues of cell-based tasks are ran,
with their priorities and dependencies determining the order that the tasks
......@@ -9,7 +12,7 @@ are ran in. Tasks are only added to a queue when they are ready to run, that
is they are not waiting for other tasks. This order also determines when the
sends and recvs needed to update data on other ranks are initiated as this
happens when the associated task is queued. The sends and recvs are considered
to be complete when MPI_Test returns true and this unlocks any dependencies
to be complete when `MPI_Test` returns true and this unlocks any dependencies
they have. Obviously a step cannot complete until all the sends and recvs are
themselves also complete, so the performance of the MPI library and lower
layers is critical. This seems to be most significant, not when we have a lot
......@@ -17,9 +20,9 @@ of work, or very little, but for intermediary busy steps, when the local work
completes much sooner than the MPI exchanges.
In SWIFT the enqueuing of tasks, thus send and recvs initiation (using
MPI_Isend and MPI_Irecv) can happen from all the available threads, but the
polling of MPI_Test is done primarily using two queues, but these can steal
work from other queues, and other queues can steal MPI_Test calls as well.
`MPI_Isend` and `MPI_Irecv`) can happen from all the available threads, but the
polling of `MPI_Test` is done primarily using two queues, but these can steal
work from other queues, and other queues can steal `MPI_Test` calls as well.
Enqueuing and processing can happen at the same time.
To keep this simple this package uses three threads to simulate all this, a
......@@ -53,9 +56,9 @@ numbers of ranks as the original run of SWIFT.
The verbose output and output log can be inspected to see what delays are
driving the elapsed time for the step. Mainly these seem to be outlier
MPI_Test calls that take tens of milliseconds.
`MPI_Test` calls that take tens of milliseconds.
A script post-process.py can be ran on the output log to pair the sends and
A script `post-process.py` can be ran on the output log to pair the sends and
recvs across the ranks. This allows the inspection of how well things like
eager exchanges are working and what effect the size of the packets has.
......
0% Loading or .
You are about to add 0 people to the discussion. Proceed with caution.
Please register or to comment