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Legion

A Data-Centric Parallel Programming System

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Performance Profiling and Tuning

After developing a functional Legion application, it is usually necessary to profile and tune for performance. This page covers techniques to achieving high performance in Legion applications.

Preliminary Steps

When performance is desired, always build Legion in release mode (i.e. DEBUG=0 make). Release mode ensures that expensive assertions and checks are disabled, and that appropriate optimization flags are used in the build. Also, remember to remove any unnecessary build flags (e.g. -DPRIVILEGE_CHECKS, -DBOUNDS_CHECKS, or -DLEGION_SPY).

Set OUTPUT_LEVEL at an appropriate level. The default (LEVEL_DEBUG) is usually acceptable as dynamic checks are sufficient to ensure that the extra logging statements are not a performance problem. However, a higher level such as LEVEL_SPEW might cause problems, even when disabled on the command-line.

If using GASNet, ensure that GASNet has been properly configured and optimized. GASNet can be important for performance on multi-node runs.

Legion Prof

Before taking steps towards optimizing a program, it is usually helpful to profile the application in question. Legion has a task-level profiler called Legion Prof which is used for this purpose.

Legion Prof is compiled into all builds by default, so no additional compile-time flags are required. However, it is always recommended to compile Legion in release mode (i.e. DEBUG=0) before measuring performance.

Legion Prof outputs its logs in a compressed binary format using ZLIB. If you don’t have ZLIB on your system, you can set USE_ZLIB=0 in your Makefile.

Generating a Profile

To profile an application, run with -lg:prof <N> where N is the number of nodes to be profiled. (N can be less than the total number of nodes—this profiles a subset of the nodes.) Use the -lg:prof_logfile <logfile> flag to save the output from each node to a separate file. For example, data from node 0 will be saved in <logfile>0.gz, data from node 1 in <logfile>1.gz, etc. Finally, pass the resulting log files to legion_prof.py.

DEBUG=0 make
./app -lg:prof <N> -lg:prof_logfile prof_log
$LG_RT_DIR/../tools/legion_prof.py prof_log*

This will generate a subdirectory called legion_prof under the current directory, including a file named index.html. Open this file in a browser.

A sample of Legion Prof’s output is shown below.

This profile shows the utilization graph of the memories and processors during the run. Legion Prof also can also display more detailed information as described in Interacting with a Profile.

Interacting with a Profile

The default view of the profile only shows overall utilization figures to display a broad overview of the performance of the application. However, after clicking on the triangles on the left had side of the screen, the profile can display more detailed information. This will uncollapse the corresponding line to show the actual events that happened during the application. For example, after clicking on the triangle for node 0 (CPU):

Hovering over a box in this more detailed view will display relevant information such as the name of the task, the start and end time, and the total duration of the task. The color of the box will be lighter if the task was waiting or ready to be scheduled, but was not actually executing.

Some of the boxes in the profile are gray. These are groups of tasks that are too small to be viewed at the current zoom level. To zoom in, click and drag the mouse over a region of interest.

After zooming in on a section, you can return to the previous zoom level by pressing u on the keyboard. You can also reset the zoom level by pressing 0. For other options, use the ? key to show a help message.

Press esc or ? to exit the help message.

One of the options in the help message was searching for tasks. To search for a task, press s, type a search query, and press enter. Legion Prof will highlight the matched tasks in red. To clear the search, press c.

Advanced Features

Legion Prof can interpret Legion Spy data to draw dependencies between tasks. This can be helpful when debugging performance issues. To take advantage of this feature, run your application with both Legion Prof and Legion Spy logging enabled. Then pass both the Legion Prof and Legion Spy traces to legion_prof.py.

DEBUG=0 make
./app -lg:prof <N> -lg:spy -logfile spy_log% -lg:prof_logfile prof_log
$LG_RT_DIR/../tools/legion_prof.py prof_log* spy_log*

After Legion Prof creates the profile, you will be able to click on tasks that have dependency data. Left-clicking will draw solid lines to tasks that depend on the current task as well as from tasks that the current task depends on. Hovering over a line will show which way the dependency flows.

Similarly, right-clicking will draw the parent/child relationships for the current task.

You can deselect the current task by either clicking on a different task or left/right clicking on the current task to remove dependency and parent/child relationship lines, respectively.

Finally, pressing a will draw the critical path of dependencies in the application. Inspecting this line can be helpful to determine where optimizations should be made to improve the performance of the application. To remove the critical path line, simply press a again.

Legion Prof Troubleshooting

If you try to access index.html locally (by opening file:///.../index.html), you may run into issues if you use Google Chrome. We recommend using Firefox or Safari, or accessing your profile over a remote web server. Alternately, you can spin up a simple web server locally:

cd legion_prof
python -m SimpleHTTPServer

and then open localhost:80000 from Chrome. One final option if you want to use Chrome on local profiles is to launch Chrome with chrome --allow-file-access-from-files from the terminal. Note that you will need to completely close Chrome before doing this.

General Optimization Techniques

In general there are three primary factors that contribute to the performance of a Legion application.

  1. Overall task and data movement throughput
  2. Latency of the critical path through the task/copy graph
  3. Runtime overhead

There are many variables which can impact each of these three performance metrics including the decisions made by the application mapper(s), the size and shape of the region trees, the selected regions and fields requested by tasks, and the settings of many of the variables specified on this page. The goal of performance profiling is to determine which variables are impacting performance and tune them for the best performance. Due to the large parameter spaces in Legion programs this is a difficult problem to solve in general. We therefore give some basic suggestions that we use when tuning Legion applications.

When tuning our Legion applications we usually make heavy use of Legion Prof to understand the costs of our mapping decisions and the performance of the runtime. A normal tuning cycle involves profiling the execution of our application, investing Legion Prof output to determine the most pressing bottlenecks, and then modifying the necessary parameters or Mapper implementations. Legion Prof time lines are the most common output that we investigate as they can show which processors are under-utilized as well as when there are bubbles in task execution. While this is useful, it doesn’t always suggest why there are bubbles. Legion Prof does not show data movement operations and therefore bubbles can be caused either by critical path dependencies, or by pending data movement operations. We currently rely on application specific information to discern the cause.

While the performance of the application is usually the limiter, it is also possible for runtime overhead to be a performance limiter. This most commonly manifests by seeing bubbles in processor executions. The real indicator is evident when looking at the runtime meta-tasks being run on the utility processors. If the runtime is not running sufficiently far ahead of the actual execution of the program, the IDs of the tasks being mapped will be in close proximity to the tasks being executed. If the runtime is running efficiently it will often be 10s to 100s of IDs in front of the actual execution.

Performance debugging, even in Legion, is still very much an art form and we are open to suggestions as to ways to improve it. We are open to suggestions about new tools to build to help with Legion debugging and are willing to place the appropriate logging calls in the Legion runtime, especially if users are interested in helping to write the post-processing and visualization tools. If you have suggestions, please post them to the mailing list.

Legion Configuration

Legion has a large number of flags that impact performance. These can be divided into two categories:

  1. Flags to configure the machine (number of CPUs, GPUs, memory sizes, etc.)
  2. Flags to configure internal runtime parameters (scheduling windows, etc.)

Machine Configuration

Legion provides a multitude of command line parameters which can be used to specify the machine configuration that is made visible to an application and its mapper through the Machine object. All these flags are interpreted by the Realm low-level runtime and are therefore prefixed with -ll:.

  • -ll:cpu <int> - Specify the number of latency optimized Processors to allocate on each node. Typically this should be one or two less than the number of physical CPU cores available on a node. The default is 1.
  • -ll:util <int> - Specify the number of utility Processors to allocate on each node. Utility processors are used for performing runtime meta-tasks such as dependence analysis and mapping. More utility processors help reduce runtime overhead at the cost of consuming more hardware cores for meta-work. The default is 1.
  • -ll:gpu <int> - Specify the number of GPUs on each node. This should be less than or equal to the number of discrete GPUs on the node. The default is 0.
  • -ll:amsg <int> - Set the number of threads used for handling active message traffic between nodes. There must always be at least one of these threads. Applications which are very communication intensive may consider increasing the number of threads for handling active messages. The default value is 1.
  • -ll:dma <int> - Specify the number of threads used for handling copies. For applications which do significant movement of data between distinct memories, increasing the number of DMA threads can significantly improve performance. There must always be at least one DMA thread. The default value is 1.
  • -ll:csize <int> - Set the amount of system memory per node available to the CPUs (e.g. DRAM) in MBs. In general this value should be less than the maximum amount of physical memory available on a node. If it is set larger, applications may end up thrashing virtual DRAM pages. The default is 512 MB.
  • -ll:gsize <int> - Specify the amount of memory that should be allocated to the GASNet global memory in MB. The GASNet memory provides a globally visible memory across all nodes. This means that all nodes can read and write the GASNet memory but at the extreme cost of doing explicit RDMA reads and writes. The amount of GASNet memory divided by the total number of nodes must be less than or equal to the GASNet segment size (e.g. set by the environment variable GASNET_PHYSMEM_MAX). The default value is 256 MB.
  • -ll:rsize <int> - Set the amount of GASNet registered memory in MB to be allocated on each node. Registered memory is pinned memory that is registered with GASNet so that it may used for doing one-sided RDMA operations. It is therefore especially useful for placing physical instances which are often communicated in these memories (e.g. ghost region instances). The maximum amount of registered memory must be less than GASNet segment size (controlled by the GASNET_PHYSMEM_MAX environment variable). The default value is 0.
  • -ll:fsize <int> - Indicate the amount of memory in MB to be allocated for GPU’s framebuffer memory. This value is required to be less than the total amount of physical GDDR5 memory available in each GPU. The default is 256 MB.
  • -ll:zsize <int> - Indicate the amount of zero-copy memory in MB to allocate on each node. Zero-copy memory is memory that is mapped into each GPUs address space as well as to CPU-side DRAM and is therefore accessible by all the CPUs and GPUs on the same node. However, it is very slow for GPUs to access because reads and writes must traverse the PCI-E bus. For more information on zero-copy memory we refer users here. The default value is 64 MB.
  • -ll:stacksize <int> - Indicate the maximum stack size to allocate for each CPU thread in MB. By default the values is 2 MB (consistent with the Posix nominal value).
  • -ll:sdpsize <int> - Set the maximum amount of pinned memory available for sending GASNet active messages on a node. Users should really only need to increase this value if they observe failures of the set_payload runtime method. The default is 64 MB.
  • -ll:lmbsize <int> - Indicate the maximum size of active messages that can be sent between a pair of nodes by fixing an upper bound on long message buffer (LMB) sizes in MB. Larger messages will get broken into chunks of at most this size. The default is 1 MB.
  • -ll:numlmbs <int> - Set the maximum number of long message buffers that should be created between a pair of nodes. More buffers will increase the number of large active messages that can be in flight simultaneously, but at the cost of increasing pinned registered memory usage in the GASNet segment. The default is 2.
  • -ll:pin <int> - Specify whether the CPU system memory allocated by the -ll:csize flag should be pinned. Pinning the memory means that copies to and from the GPU can make use of the asynchronous DMA engines on the GPU. Note that pinning doesn’t register this memory with GASNet which distinguishes this memory from registered memory. The default value is 1 indicating true.

Legion Runtime Performance Flags

The Legion runtime itself also provides command-line flags for controlling the execution of its out-of-order task processor. Not surprisingly many of these flags are related to parameters which are tuned by hardware architects in out-of-order processors. Since Legion is built in software though, users have the direct ability to tune these parameters to their particular application. In keeping with the naming scheme used above, all Legion runtime flags are prefixed by -lg:.

  • -lg:window - Specify the maximum number of operations that can be outstanding in a given task context (e.g. how many sub-tasks and operations can a task issue) before the task is stalled. This upper bound provides one way of applying back pressure on long running tasks and prevent them from running too far in advance. In many ways this value is analogous to the size of a re-order buffer (ROB) in hardware processors. The default value is 1024.
  • -lg:sched - Set the maximum number of ready tasks which must be available to run on a low-level processor before the high-level runtime stops invoking the mapper to schedule more tasks. This controls how far in advance tasks are scheduled onto a processor. Note by Little’s Law, more tasks must be mapped to a processor if the tasks require longer to map. In many ways this value is analogous to the number of entries available at reservation stations for hardware resource units. The default value is 1.
  • -lg:width - Indicate the number of operations which the high-level runtime should consider when making a scheduling pass. The wider this is, the more operations will be touched in a scheduling pass. This can increase throughput, but also increase latency of scheduling on processors. This value is analogous to the super-scalar width of modern processors. The default value is 4.
  • -lg:message - Specify the maximum size of messages in bytes that can be sent between two instances of the high-level runtime. Smaller message sizes can decrease the latency of communication by pipelining communication, but can achieve lower interconnect bandwidth. Larger messages can achieve higher bandwidth, but at the cost of higher latency. The default value is 4096 bytes to ensure reasonable latency and bandwidth by ensuring medium size GASNet active messages are used on both Infiniband and Cray GASNet conduits.
  • -lg:filter - This flag is more subtle and we only encourage users to make use of it after gaining some understanding of how the Legion runtime operates. In some application, there are many live operations that are non-interfering in different ways simultaneously. Since most non-interference tests end up failing, there is significant amounts of runtime analysis that is not useful. To help reduce this overhead, users can request that the list of users of physical instances be trimmed to some maximum size, possible in a way that reduces the precision of runtime analysis but not in a way that is unsound. By default this setting is disabled with the value 0. Reasonable values are usually in the range of 128 to 4096.