This tutorial will help you get started with writing parallel programs in
Multicore OCaml. All the code examples along with their corresponding dune file
are available in the code/ directory. The tutorial is organised into the
following sections:
Multicore OCaml is an extension of OCaml with native support for Shared-Memory
Parallelism (SMP) through Domains and Concurrency through Algebraic Effects.
It is merged to trunk OCaml. OCaml 5.0 will be the first release to officially
support Multicore.
Concurrency is how we partition multiple computations such that they can run in overlapping time periods rather than strictly sequentially. Parallelism is the act of running multiple computations simultaneously, primarily by using multiple cores on a multicore machine. The Multicore Wiki has comprehensive notes on the design decisions and current status of Concurrency and Parallelism in Multicore OCaml.
The Multicore OCaml compiler ships with a concurrent major and a stop-the-world minor garbage collector (GC). The parallel minor GC doesn't require any changes to the C API, thereby not breaking any associated code with C API. OCaml 5.0 is expected to land with support for Shared-Memory Parallelism and Algebraic Effects. A historical variant of the Multicore minor garbage collector is the concurrent minor collector. Benchmarking experiments showed better results in terms of throughput and latency on the stop-the-world parallel minor collector, hence that's chosen to be the default minor collector on Multicore OCaml, and the concurrent minor collector is not actively developed. For the intrigued, details on the design and evaluation of the Multicore GC and compiler are in our academic publications.
The Multicore ecosystem also has the following libraries to complement the compiler:
- Domainslib: data and control structures for parallel programming
- Eio: effects-based direct-style IO for multicore OCaml
- Saturn: lock-free data structures (list, hash, bag and queue)
- Reagents: composable lock-free concurrency library for expressing fine grained parallel programs on Multicore OCaml
- Kcas: software transactional memory (STM) implementation based on an atomic lock-free multi-word compare-and-set (MCAS) algorithm
Find ways to profitably write parallel programs in Multicore OCaml. The reader is assumed to be familiar with OCaml. If not, they are encouraged to read Real World OCaml. The effect handlers' story is not covered here. For anyone interested, please check out this tutorial and some examples.
Instructions to install OCaml 5 compiler is here.
It will also be useful to install utop on your Multicore switch by running
opam install utop, which should work out of the box.
Domains are the basic unit of Parallelism in Multicore OCaml.
let square n = n * n
let x = 5
let y = 10
let _ =
let d = Domain.spawn (fun _ -> square x) in
let sy = square y in
let sx = Domain.join d in
Printf.printf "x = %d, y = %d\n" sx syDomain.spawn creates a new execution process that runs along with the
current domain.
Domain.join d blocks until the domain d runs to completion. If the domain
returns a result after its execution, Domain.join d also returns that value.
If it raises an uncaught exception, that is thrown. When the parent domain
terminates, all other domains also terminate. To ensure that a domain runs to
completion, we have to join the domain.
Note that the square of x is computed in a new domain and that of y in the parent domain.
To create its corresponding dune file, run this code:
(executable
(name square_domain)
(modules square_domain))
Make sure to use a Multicore switch to build this and all other subsequent examples in this tutorial.
To execute the code:
$ dune build square_domain.exe
$ ./_build/default/square_domain.exe
x = 25, y = 100
As expected, the squares of x and y are 25 and 100.
Common Error Message
Some common errors while compiling Multicore code are:
Error: Unbound module Domain
Error: Unbound module Atomic
Error: Library "domainslib" not found.
These errors usually mean that the compiler switch used is not a Multicore switch. Using a Multicore compiler variant should resolve them.
Domainslib is a parallel programming library for Multicore OCaml. It provides
the following APIs which enable easy ways to parallelise OCaml code with only a few
modifications to sequential code:
- Task: Work stealing task pool with async/await Parallelism and
parallel_{for, scan} - Channels: Multiple Producer Multiple Consumer channels which come in two flavours—bounded and unbounded
Domainslib is effective in scaling performance when parallelisable
workloads are available.
In the Domains section, we saw how to run programs on multiple cores by
spawning new domains. We often find ourselves spawning and joining
new domains numerous times in the same program, if we were to use that approach
for executing code in parallel. Creating new domains is an expensive operation, so
we should attempt to limit those when possible. Task.pool allows
execution of all parallel workloads in the same set of domains spawned at
the beginning of the program. Here is how they work:
Note: If you are running this on utop, run #require "domainslib" with the hash before this.
# open Domainslib
# let pool = Task.setup_pool ~num_domains:3 ()
val pool : Task.pool = <abstr>We have created a new task pool with three new domains. The parent domain is
also part of this pool, thus making it a pool of four domains. After the pool is
setup, we can use it to execute all tasks we want to run in parallel. The
setup_pool function requires us to specify the number of new domains to be
spawned in the task pool. Ideally, the number of domains used to initiate a task pool
will match the number of available cores. Since the parent domain also
takes part in the pool, the num_domains parameter should be one
less than the number of available cores.
Although not strictly necessary, we highly recommended closing the task pool after execution of all tasks. This can be done as follows:
# Task.teardown_pool poolThis deactivates the pool, so it's no longer usable. Make sure to do this only after all tasks are done.
In the Task API, a powerful primitive called parallel_for can be used to
parallelise computations used in for loops. It scales well with very little
change to the sequential code.
Consider the example of matrix multiplication.
First, write the sequential version of a function which performs matrix multiplication of two matrices and returns the result:
let matrix_multiply a b =
let i_n = Array.length a in
let j_n = Array.length b.(0) in
let k_n = Array.length b in
let res = Array.make_matrix i_n j_n 0 in
for i = 0 to i_n - 1 do
for j = 0 to j_n - 1 do
for k = 0 to k_n - 1 do
res.(i).(j) <- res.(i).(j) + a.(i).(k) * b.(k).(j)
done
done
done;
resTo make this function run in parallel, one might be inclined to spawn a new domain for every iteration in the loop, which would look like:
let domains = Array.init i_n (fun i ->
Domain.spawn(fun _ ->
for j = 0 to j_n - 1 do
for k = 0 to k_n - 1 do
res.(i).(j) <- res.(i).(j) + a.(i).(k) * b.(k).(j)
done
done)) in
Array.iter Domain.join domainsThis will be disastrous in terms of performance, mostly because spawning a new domain is an expensive operation. Alternatively, a task pool offers a finite set of available domains that can be used to run your computations in parallel.
Arrays are usually more efficient compared with lists in Multicore OCaml. Although they are not generally favoured in functional programming, using arrays for the sake of efficiency is a reasonable trade-off.
A better way to parallelise matrix multiplication is with the help of a
parallel_for.
let parallel_matrix_multiply pool a b =
let i_n = Array.length a in
let j_n = Array.length b.(0) in
let k_n = Array.length b in
let res = Array.make_matrix i_n j_n 0 in
Task.parallel_for pool ~start:0 ~finish:(i_n - 1) ~body:(fun i ->
for j = 0 to j_n - 1 do
for k = 0 to k_n - 1 do
res.(i).(j) <- res.(i).(j) + a.(i).(k) * b.(k).(j)
done
done);
resObserve quite a few differences between the parallel and sequential
versions: The parallel version takes an additional parameter pool because
the parallel_for executes the for loop on the domains present in
that task pool. While it is possible to initialise a task pool inside the
function itself, it's always better to have a single task pool used across the
entire program. As mentioned earlier, this is to minimise the cost involved in
spawning a new domain. It's also possible to create a global task pool to use
across, but for the sake of reasoning better about your code, it's recommended
to use it as a function parameter.
Let's examine the parameters of parallel_for. It takes in
pool, as discussed earlierstartandfinish, as the names suggest, are the starting and ending values of the loop iterationsbodycontains the actual loop body to be executed
parallel_for also has an optional parameter: chunk_size, which determines the
granularity of tasks when executing on multiple domains. If no parameter
is given for chunk size, the program determines a default chunk size that performs
well in most cases. Only if the default chunk size doesn't work well is it
recommended to experiment with different chunk sizes. The ideal chunk_size
depends on a combination of factors:
-
Nature of the Loop: There are two things to consider pertaining to the loop when deciding on a
chunk_size—the number of iterations in the loop and the amount of time each iteration takes. If the amount of time is roughly equal, then thechunk_sizecould be the number of iterations divided by the number of cores. On the other hand, if the amount of time taken is different for every iteration, the chunks should be smaller. If the total number of iterations is a sizeable number, achunk_sizelike 32 or 16 is safe to use, whearas if the number of iterations is low, like 10, achunk_sizeof 1 would perform best. -
Machine: Optimal chunk size varies across machines, so it's recommended to experiment with a range of values to find out what works best on yours.
Let's find how the parallel matrix multiplication scales on multiple cores.
Speedup
The speedup vs. core is enumerated below for input matrices of size 1024x1024:
| Cores | Time (s) | Speedup |
|---|---|---|
| 1 | 9.172 | 1 |
| 2 | 4.692 | 1.954816709 |
| 4 | 2.293 | 4 |
| 8 | 1.196 | 7.668896321 |
| 12 | 0.854 | 10.74004684 |
| 16 | 0.76 | 12.06842105 |
| 20 | 0.66 | 13.8969697 |
| 24 | 0.587 | 15.62521295 |
We've achieved a speedup of 16 with the help of a parallel_for. It's very
much possible to achieve linear speedups when parallelisable workloads are
available.
Note that parallel code performance heavily depends on the machine. Some machine settings specific to Linux systems for obtaining optimal results are described here.
The parallel_for has an implicit barrier, meaning any other tasks
waiting to be executed in the same pool will start only after all chunks
in the parallel_for are complete, so we need not worry about creating and
inserting barriers explicitly between two parallel_for loops (or some other
operation) after a parallel_for. Consider this scenario: we have three
matrices m1, m2, and m3. We want to compute (m1*m2) * m3, where *
indicates matrix multiplication. For the sake of simplicity, let's assume all
three are square matrices of the same size.
let parallel_matrix_multiply_3 pool m1 m2 m3 =
let size = Array.length m1 in
let t = Array.make_matrix size size 0 in (* stores m1*m2 *)
let res = Array.make_matrix size size 0 in
Task.parallel_for pool ~start:0 ~finish:(size - 1) ~body:(fun i ->
for j = 0 to size - 1 do
for k = 0 to size - 1 do
t.(i).(j) <- t.(i).(j) + m1.(i).(k) * m2.(k).(j)
done
done);
Task.parallel_for pool ~start:0 ~finish:(size - 1) ~body:(fun i ->
for j = 0 to size - 1 do
for k = 0 to size - 1 do
res.(i).(j) <- res.(i).(j) + t.(i).(k) * m3.(k).(j)
done
done);
resIn a hypothetical situation where parallel_for didn't have an implicit
barrier, as in the example above, it's very likely that the computation of res
wouldn't be correct. Since we already have an implicit barrier, it will perform
the right computation.
for i = start to finish do
<body>
done
A sequential for loop, like the one above, runs its iterations in the exact
same order, from start to finish. However, parallel_for makes the order of
execution arbitrary and varies it between two runs of the exact same code. If
the iteration order is important for your code, it's
advisable to use parallel_for with some caution.
If there are any dependencies within the loop, such as a current iteration
depending on the result of a previous iteration, odds are very high that the
correctness of the code no longer holds if parallel_for is used. Task API has
a primitive parallel_scan which might come in handy in scenarios like this.
A parallel_for loop easily parallelises iterative tasks. Async-Await offers more
flexibility to execute parallel tasks, which is especially useful in
recursive functions. Earlier we saw how to setup and tear down a task
pool. The Task API also has the facility to run specific tasks on a task pool.
To calculate a Fibonacci Sequence in parallel, first write a sequential function to calculate Fibonacci numbers. The following is a naive Fibonacci function without tail-recursion:
let rec fib n =
if n < 2 then 1
else fib (n - 1) + fib (n - 2)Observe that the two operations in recursive case fib (n - 1) and fib (n -2)
do not have any mutual dependencies, which makes it convenient to
compute them in parallel. Essentially, we can calculate fib (n - 1) and fib (n - 2)
in parallel and then add the results to get the answer.
Do this by spawning a new domain for performing the calculation and joining it to obtain the result. Be careful to not spawn more domains than number of cores available.
let rec fib_par n d =
if d <= 1 then fib n
else
let a = fib_par (n-1) (d-1) in
let b = Domain.spawn (fun _ -> fib_par (n-2) (d-1)) in
a + Domain.join bWe can also use task pools to execute tasks asynchronously, which is less tedious and scales better.
let rec fib_par pool n =
if n <= 40 then fib n
else
let a = Task.async pool (fun _ -> fib_par pool (n-1)) in
let b = Task.async pool (fun _ -> fib_par pool (n-2)) in
Task.await pool a + Task.await pool bNote some differences from the sequential version of Fibonacci:
-
pool—> an additional parameter for the same reasons inparallel_for -
if n <= 40 then fib n-> when the input is less than 40, run the sequentialfibfunction. When the input number is small enough, it's better to perform the calculations sequentially. We've taken40as the threshold (above). Some experimentation would help find an acceptible threshold, below which the computation can be performed sequentially. -
Task.asyncandTask.await-> used to run the tasks in parallel-
Task.async executes the task in the pool asynchronously and returns a promise, a computation that is not yet complete. After the execution finishes, it result will be stored in the promise.
-
Task.await waits for the promise to complete its execution. Once it's done, it returns the result of the task. In case the task raises an uncaught exception,
awaitalso raises the same exception.
-
Channels act as a medium to communicate data between domains and can be shared between multiple sending and receiving domains. Channels in Multicore OCaml come in two flavours:
-
Bounded: buffered channels with a fixed size. A channel with the buffer size 0 corresponds to a synchronised channel, and buffer size 1 gives the
MVarstructure. Bounded channels can be created with any buffer size. -
Unbounded: unbounded channels have no limit on the number of objects they can hold, so they are only constrained by memory availability.
open Domainslib
let c = Chan.make_bounded 0
let _ =
let send = Domain.spawn(fun _ -> Chan.send c "hello") in
let msg = Chan.recv c in
Domain.join send;
Printf.printf "Message: %s\n" msgIn the above example, we have a bounded channel c of size 0. Any send to the channel will be blocked
until a corresponding receive (recv) is encountered. So, if we
remove the recv, the program would be blocked indefinitely.
open Domainslib
let c = Chan.make_bounded 0
let _ =
let send = Domain.spawn(fun _ -> Chan.send c "hello") in
Domain.join send;The above example would block indefinitely because the send
does not have a corresponding recv. If we instead create a bounded channel
with buffer size n, it can store up to [n] objects in the channel without a
corresponding receive, exceeding which the sending would block. We can try it
with the same example as above by changing the buffer size to 1:
open Domainslib
let c = Chan.make_bounded 1
let _ =
let send = Domain.spawn(fun _ -> Chan.send c "hello") in
Domain.join send;Now the send will not block anymore.
If you don't want to block in send or recv, send_poll and recv_poll might
come in handy. They return a Boolean value, so if the operation was successful we
get a true, otherwise a false.
open Domainslib
let c = Chan.make_bounded 0
let _ =
let send = Domain.spawn(fun _ ->
let b = Chan.send_poll c "hello" in
Printf.printf "%B\n" b) in
Domain.join send;Here the buffer size is 0 and the channel cannot hold any object, so this program prints a false.
The same channel may be shared by multiple sending and receiving domains.
open Domainslib
let num_domains = try int_of_string Sys.argv.(1) with _ -> 4
let c = Chan.make_bounded num_domains
let send c =
Printf.printf "Sending from: %d\n" (Domain.self () :> int);
Chan.send c "howdy!"
let recv c =
Printf.printf "Receiving at: %d\n" (Domain.self () :> int);
Chan.recv c |> ignore
let _ =
let senders = Array.init num_domains
(fun _ -> Domain.spawn(fun _ -> send c )) in
let receivers = Array.init num_domains
(fun _ -> Domain.spawn(fun _ -> recv c)) in
Array.iter Domain.join senders;
Array.iter Domain.join receivers(Domain.self () :> int) returns the id of current domain.
Now that we have some idea about how channels work, let's consider a more realistic example by writing a generic task distributor that executes tasks on multiple domains:
module C = Domainslib.Chan
let num_domains = try int_of_string Sys.argv.(1) with _ -> 4
let n = try int_of_string Sys.argv.(2) with _ -> 100
type 'a message = Task of 'a | Quit
let c = C.make_unbounded ()
let create_work tasks =
Array.iter (fun t -> C.send c (Task t)) tasks;
for _ = 1 to num_domains do
C.send c Quit
done
let rec worker f () =
match C.recv c with
| Task a ->
f a;
worker f ()
| Quit -> ()
let _ =
let tasks = Array.init n (fun i -> i) in
create_work tasks ;
let factorial n =
let rec aux n acc =
if (n > 0) then aux (n-1) (acc*n)
else acc in
aux n 1
in
let results = Array.make n 0 in
let update r i = r.(i) <- factorial i in
let domains = Array.init (num_domains - 1)
(fun _ -> Domain.spawn(worker (update results))) in
worker (update results) ();
Array.iter Domain.join domains;
Array.iter (Printf.printf "%d ") resultsWe have created an unbounded channel c which acts as a store for all
tasks. We'll pay attention to two functions here: create_work and worker.
create_work takes an array of tasks and pushes all task elements to the
channel c. The worker function receives tasks from the channel and executes
a function f with the received task as a parameter. It keeps repeating until it
encounters a Quit message, which indicates worker can terminate.
Use this template to run any task on multiple cores by running the
worker function on all domains. This example runs a naive factorial
function. The granularity of a task could also be tweaked by changing it in
the worker function. For instance, worker can run for a range of tasks instead
of single one.
While writing parallel programs in Multicore OCaml, it's quite common to
encounter overheads that might deteriorate the code's performance. This
section describes ways to discover and fix those overheads. Within the Multicore runtime,
Linux commands perf and eventlog are particularly useful tools for
performance debugging. Let's do that with the help of an example:
The Linux perf tool has proven to be very useful when profiling Multicore
OCaml code.
Profiling Serial Code
Profiling serial code can help identify parts of code that can potentially benefit from parallelising. Let's do it for the sequential version of matrix multiplication:
perf record --call-graph dwarf ./matrix_multiplication.exe 1024
This results in a profile showing how much time is spent in the matrix_multiply
function, which we wanted to parallelise. Remember, if a lot more time is spent
outside the function we'd like to parallelise,
the maximum speedup possible to achieve would be lower.
Profiling serial code can help reveal the hotspots where we might want to introduce parallelism.
Samples: 51K of event 'cycles:u', Event count (approx.): 28590830181
Children Self Command Shared Object Symbol
+ 99.84% 0.00% matmul.exe matmul.exe [.] caml_start_program
+ 99.84% 0.00% matmul.exe matmul.exe [.] caml_program
+ 99.84% 0.00% matmul.exe matmul.exe [.] camlDune__exe__Matmul__entry
+ 99.32% 99.31% matmul.exe matmul.exe [.] camlDune__exe__Matmul__matrix_multiply_211
+ 0.57% 0.04% matmul.exe matmul.exe [.] camlStdlib__array__init_104
0.47% 0.37% matmul.exe matmul.exe [.] camlStdlib__random__intaux_278
Linux perf can be helpful when identifying overheads in parallel code, which can improve
the performance by removing overheads.
Parallel Initialisation of a Float Array with Random Numbers
Array initialisation using the standard library's Array.init is sequential.
A program's parallel workloads scale according to the number of cores
used, although the initialisation takes the same amount of time in all cases.
This might become a bottleneck for parallel workloads.
For float arrays, we have Array.create_float to create a fresh float
array. Use this to allocate an array and perform the initialisation in
parallel. Let's do the initialisation of a float array with random numbers in
parallel.
Naive Implementation
Below is a naive implementation that will initialise all array elements with a Random number:
open Domainslib
let num_domains = try int_of_string Sys.argv.(1) with _ -> 4
let n = try int_of_string Sys.argv.(2) with _ -> 100000
let a = Array.create_float n
let _ =
let pool = Task.setup_pool ~num_domains:(num_domains - 1) () in
Task.run pool (fun () -> Task.parallel_for pool ~start:0
~finish:(n - 1) ~body:(fun i -> Array.set a i (Random.float 1000.)));
Task.teardown_pool poolMeasure how it scales:
| #Cores | Time(s) |
|---|---|
| 1 | 3.136 |
| 2 | 10.19 |
| 4 | 11.815 |
Although we expected to see speedup executing in multiple cores, the code actually slows down as the number of cores increase. There's something unnoticably wrong with the code.
Let's profile the performance with the Linux perf profiler:
$ perf record ./_build/default/float_init_par.exe 4 100_000_000
$ perf report
The perf report would look something like this:
The overhead at Random bits is a whopping 87.99%! Typically there's no single cause that we can attribute to such overheads, since they are very specific to the program. It might need a little careful inspection to find out what is causing them. In this case, the Random module shares the same state amongst all domains, which causes contention when multiple domains are trying to access it simultaneously.
To overcome that, use a different state for every domain so there isn't any contention from a shared state.
module T = Domainslib.Task
let n = try int_of_string Sys.argv.(2) with _ -> 1000
let num_domains = try int_of_string Sys.argv.(1) with _ -> 4
let arr = Array.create_float n
let _ =
let domains = T.setup_pool ~num_domains:(num_domains - 1) () in
let states = Array.init num_domains (fun _ -> Random.State.make_self_init()) in
T.run domains (fun () -> T.parallel_for domains ~start:0 ~finish:(n-1)
~body:(fun i ->
let d = (Domain.self() :> int) mod num_domains in
Array.unsafe_set arr i (Random.State.float states.(d) 100. )))We have created num_domains different Random States, each to be used by a different domain. This might come
across as a hack, but if it helps achieve better performance, there is no harm in using them,
as long as the correctness is intact.
Let's run this on multiple cores:
| #Cores | Time(s) |
|---|---|
| 1 | 3.828 |
| 2 | 3.641 |
| 4 | 3.119 |
Examining the times, though it is not as bad as the previous case, it isn't
close to what we expected. Here's the perf report:
The overheads at Random bits is less than the previous case, but it's still
quite high at 59.73%. We've used a separate Random State for every domain, so
the overheads aren't caused by any shared state; however, if we look closely, the
Random States are all allocated by the same domain in an array with a small
number of elements, possibly located close to each other in physical memory.
When multiple domains try to access them, they might be sharing cache
lines, or false sharing. We can confirm our suspicion with the
help of perf c2c on Intel machines:
$ perf c2c record _build/default/float_init_par2.exe 4 100_000_000
$ perf c2c report
Shared Data Cache Line Table (2 entries, sorted on Total HITMs)
----------- Cacheline ---------- Total Tot ----- LLC Load Hitm ----- ---- Store Reference ---- --- Loa
Index Address Node PA cnt records Hitm Total Lcl Rmt Total L1Hit L1Miss Lc
0 0x7f2bf49d7dc0 0 11473 13008 94.23% 1306 1306 0 1560 595 965 ◆
1 0x7f2bf49a7b80 0 271 368 5.48% 76 76 0 123 76 47
As evident from the report, there's quite a considerable amount of false sharing happening in the code. To eliminate false sharing, allocate the Random State in the domain that is going to use it, so the states will be allocated with memory locations far from each other.
module T = Domainslib.Task
let n = try int_of_string Sys.argv.(2) with _ -> 1000
let num_domains = try int_of_string Sys.argv.(1) with _ -> 4
let arr = Array.create_float n
let init_part s e arr =
let my_state = Random.State.make_self_init () in
for i = s to e do
Array.unsafe_set arr i (Random.State.float my_state 100.)
done
let _ =
let domains = T.setup_pool ~num_domains:(num_domains - 1) () in
T.run domains (fun () -> T.parallel_for domains ~chunk_size:1 ~start:0 ~finish:(num_domains - 1)
~body:(fun i -> init_part (i * n / num_domains) ((i+1) * n / num_domains - 1) arr));
T.teardown_pool domainsNow the results are:
| Cores | Time | Speedup |
|---|---|---|
| 1 | 3.055 | 1 |
| 2 | 1.552 | 1.968427835 |
| 4 | 0.799 | 3.823529412 |
| 8 | 0.422 | 7.239336493 |
| 12 | 0.302 | 10.11589404 |
| 16 | 0.242 | 12.62396694 |
| 20 | 0.208 | 14.6875 |
| 24 | 0.186 | 16.42473118 |
In this process, we have essentially identified bottlenecks for scaling and
eliminated them to achieve better speedups. For more details on profiling with
perf, please refer these notes.
The Multicore runtime supports OCaml instrumented runtime. The instrumented runtime enables capturing metrics about various GC activities. Eventlog-tools is a library that provides tools to parse the instrumentation logs generated by the runtime. Some handy tools are described in the README.
Eventlog tools can be useful for optimizing Multicore programs.
Identify Large Pausetimes
Identifying and fixing events that cause maximum latency can improve the overall
throughput of the program. ocaml-eventlog-pausetimes displays statistics from
the generated trace files. For Multicore programs, every domain has its own
trace file, and all of them need to be fed into the input.
$ ocaml-eventlog-pausetimes caml-10599-0.eventlog caml-10599-2.eventlog caml-10599-4.eventlog caml-10599-6.eventlog
{
"name": "caml-10599-6.eventlog",
"mean_latency": 78328,
"max_latency": 5292643,
"distr_latency": [85,89,104,231,303,9923,117639,145118,179488,692880,2728990]
}
Diagnose Imbalance in Task Distribution
Eventlog can be useful to find imbalance in task distribution
in a parallel program. Imbalance in task distribution essentially means that
not all domains are provided with equal amount of computation to perform, so some
domains take longer than others to finish their computations, while the idle domains
keep waiting. This can occur when a sub-
optimal chunk_size is picked in a parallel_for.
Time periods show when an idle domain is recorded as domain/idle_wait in the
eventlog. Here is an example eventlog generated by a program with unbalanced
task distribution.
If we zoom in further, we see many domain/idle_wait events.
So far we've only found an imbalance in task distribution in the code, so we'll need to change our code accordingly to make the task distribution more balanced, which could increase the speedup.
Performance debugging can be quite tricky at times, so if you could use some help in debugging your Multicore OCaml code, feel free to create an Issue in the Multicore OCaml issue tracker along with a minimal code example.