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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<link rel="stylesheet" href="../common-revealjs/css/reveal.css">
<link rel="stylesheet" href="../common-revealjs/css/theme/white.css">
<link rel="stylesheet" href="../common-revealjs/css/custom.css">
<script>
// This is needed when printing the slides to pdf
var link = document.createElement( 'link' );
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = window.location.search.match( /print-pdf/gi ) ? '../common-revealjs/css/print/pdf.css' : '../common-revealjs/css/print/paper.css';
document.getElementsByTagName( 'head' )[0].appendChild( link );
</script>
<script>
// This is used to display the static images on each slide,
// See global-images in this html file and custom.css
(function() {
if(window.addEventListener) {
window.addEventListener('load', () => {
let slides = document.getElementsByClassName("slide-background");
if (slides.length === 0) {
slides = document.getElementsByClassName("pdf-page")
}
// Insert global images on each slide
for(let i = 0, max = slides.length; i < max; i++) {
let cln = document.getElementById("global-images").cloneNode(true);
cln.removeAttribute("id");
slides[i].appendChild(cln);
}
// Remove top level global images
let elem = document.getElementById("global-images");
elem.parentElement.removeChild(elem);
}, false);
}
})();
</script>
</head>
<body>
<div class="reveal">
<div class="slides">
<div id="global-images" class="global-images">
<img src="../common-revealjs/images/sycl_academy.png" />
<img src="../common-revealjs/images/sycl_logo.png" />
<img src="../common-revealjs/images/trademarks.png" />
<img src="../common-revealjs/images/codeplay.png" />
</div>
<!--Slide 1-->
<section class="hbox">
<div class="hbox" data-markdown>
## Multiple Devices
</div>
</section>
<!--Slide 2-->
<section class="hbox" data-markdown>
## Learning Objectives
* Learn about contexts and what they are used for
* Learn about how create dependencies across devices
* Learn about moving data between devices
</section>
<!--Slide 3-->
<section>
<div class="hbox" data-markdown>
#### What is a context
</div>
<div class="container">
<div class="col-right-1" data-markdown>
* In SYCL the underlying execution and memory resources of a platform and its devices is managed by creating a context
* A context represents one or more devices, but all devices must be associated with the same platform
</div>
</div>
</section>
<!--Slide 4-->
<section>
<div class="hbox" data-markdown>
#### Implicit context
</div>
<div class="container">
<div class="col-left-1" data-markdown>
</div>
<div class="col-right-1" data-markdown>
* Every `queue` requires a `context` to manage
memory allocation and data movement.
* If one is not specified explicitly a `queue` will
create a `context` implicitly.
</div>
</div>
</section>
<!--Slide 5-->
<section>
<div class="hbox" data-markdown>
#### Shared context
</div>
<div class="container">
<div class="col-left-1" data-markdown>
</div>
<div class="col-right-1" data-markdown>
* In order to ensure data is efficiently moved
between devices in the same platform you can create
a common `context`.
</div>
</div>
</section>
<!--Slide 6-->
<section>
<div class="hbox" data-markdown>
#### Creating queues
</div>
<div class="container">
<div class="col-left-1" data-markdown>
</div>
<div class="col-right-1" data-markdown>
* You can then create a `queue` for each of the
devices from the common `context`.
</div>
</div>
</section>
<!--Slide 7-->
<section>
<div class="hbox" data-markdown>
#### Creating an implicit context
</div>
<div class="container">
<code><pre>
auto defaultQueue = queue{};
</code></pre>
</div>
<div class="smaller-size-font" data-markdown>
* A default constructed queue object will use the
`default_selector` to choose a device and create an
implicit `context`.
</div>
</section>
<!--Slide 8-->
<section>
<div class="hbox" data-markdown>
#### Creating a context from devices
</div>
<div class="container">
<code><pre>
auto sharedContext = context{{cpuDevice, gpuDevice}};
</code></pre>
</div>
<div class="smaller-size-font" data-markdown>
* You can construct a `context` from a `std::vector` of
`device`s.
</div>
</section>
<!--Slide 9-->
<section>
<div class="hbox" data-markdown>
#### Creating a context from a platform
</div>
<div class="container">
<code><pre>
auto sharedContext = context{intelPlatform};
</code></pre>
</div>
<div class="smaller-size-font" data-markdown>
* You can construct a `context` from a `platform` in
which case it will be associated with all of the devices
of that `platform`.
</div>
</section>
<!--Slide 10-->
<section>
<div class="hbox" data-markdown>
#### Targeting multiple devices
</div>
<div class="container">
<div class="col-right-1" data-markdown>
* A single SYCL application will often want to
target multiple different devices.
* This can be useful for task level parallelism and
load balancing.
* When doing so you want to ensure that data is
moved between devices in a `context` efficiently.
</div>
</div>
</section>
<!--Slide 11-->
<section>
<div class="hbox" data-markdown>
#### Moving between devices
</div>
<div class="container" data-markdown>
* Often in heterogeneous applications it's necessary to
move data from one device to another.
* In the USM model this is done explicit via `memcpy` as
we've seen before.
* In the buffer/accessor model this is done
automatically based on dependency analysis.
</div>
</section>
<!--Slide 12-->
<section>
<div class="hbox" data-markdown>
#### Accessing data on a device
</div>
<div class="container">
<div class="col" data-markdown>
* Remember that a `buffer` will move data to a
device when required by an `accessor`.
</div>
<div class="col" data-markdown>
</div>
</div>
</section>
<!--Slide 13-->
<section>
<div class="hbox" data-markdown>
#### Accessing data on another device (same context)
</div>
<div class="container">
<div class="col" data-markdown>
* Now if a `buffer` is accessed on a device when the
latest copy of the data is on another device, the
data will be moved between the devices.
* If the two devices are of the same context the
data can be copied directly.
</div>
<div class="col" data-markdown>
</div>
</div>
</section>
<!--Slide 14-->
<section>
<div class="hbox" data-markdown>
#### Accessing data on another device (different context)
</div>
<div class="container">
<div class="col" data-markdown>
* If the devices are of different `context`s the
data must be copied via host memory.
* It's important to consider this as it could incur
further overhead when moving data between devices.
</div>
<div class="col" data-markdown>
</div>
</div>
</section>
<!--Slide 15-->
<section>
<div class="hbox" data-markdown>
#### Moving between devices (buffer/accessor)
</div>
<div class="container">
<div class="col" data-markdown>
</div>
<div class="col" data-markdown>
* If a `buffer` is accessed by kernel functions in two different devices commands are enqueued to automatically move the data to the devices it is being accessed on.
* If both of those devices are associated with the same context (i.e. same vendor) then the copy is direct.
* Otherwise the copy will generally go via the host and has additional overhead.
</div>
</div>
</section>
<!--Slide 16-->
<section>
<div class="hbox" data-markdown>
## Questions
</div>
</section>
<!--Slide 17-->
<section>
<div class="hbox" data-markdown>
#### Exercise
</div>
<div class="container" data-markdown>
Code_Exercises/Multiple_Devices/source
</div>
<div class="container" data-markdown>
Write a SYCL application that splits up a task between
two devices.
</div>
</section>
</div>
</div>
<script src="../common-revealjs/js/reveal.js"></script>
<script src="../common-revealjs/plugin/markdown/marked.js"></script>
<script src="../common-revealjs/plugin/markdown/markdown.js"></script>
<script src="../common-revealjs/plugin/notes/notes.js"></script>
<script>
Reveal.initialize({mouseWheel: true, defaultNotes: true});
Reveal.configure({ slideNumber: true });
</script>
</body>
</html>