Hello,
What Scott suggests should help a lot.
Using a slightly different approach, if you know how many tiles you need, say
60k, isn't it enough to figure out the tile position already?
Given an index 0 <= i < 60k, you should be able to map it to a 2-dimensional
coordinates if you know how many tiles you have horizontally and vertically
(actually, just horizontally is enough) - and you do, because the amount of
horizontal tiles = image.width() / tile.width().
So, if I think about it right, i / (n horizontal tiles) gives you row number,
i % (n horizontal tiles) gives you column number.
Now you can generate a 60k size vector of indices from 0 to 59999 (basically
std::iota()) and use that as an input for the tile calculation function through
QtConcurrent. Internally, you figure out the tile position in 2d space and from
that you get your QPoint + QSize for the square patch (guessing, that i index
would basically be a hand-written alternative to blockIdx in CUDA and friends).
This now allows you to calculate your tiles vector in parallel as well (without
actually using a vector).
On a side note, doesn't QtConcurrent provide a way to execute a function on a
range? Maybe you don't even need to supply a vector of indices, just a
half-open interval from 0 to 60k.
Given that QtConcurrent should handle any sort of callable, you can also supply
it a lambda with capture list containing your hyper parameters like
tile.width(), tile.height(), and any other external context information.
--
Best Regards,
Andrei
________________________________
From: Interest <interest-boun...@qt-project.org> on behalf of Scott Bloom
<sc...@towel42.com>
Sent: Monday, January 31, 2022 4:09 AM
To: Murphy, Sean <sean.mur...@centauricorp.com>; Tony Rietwyk
<t...@rightsoft.com.au>; interest@qt-project.org <interest@qt-project.org>
Subject: Re: [Interest] [External]Re: How to get QtConcurrent to do what I want?
Couple things I would try.
First, preallocate the size of the vector, or use a list if you don’t need
random access into it.
Second, just send, pos, size into the tile. Only save the values in the
constrctor. When the worker thread kicks off on a tile, then initialize it and
do any computation. This includes the allocation of the submatrix.
Also, does it need to be a shared pointer? Its clear who owns the pointers,
they aren’t being “shared” as much as use. For me, I only use shared pointers,
when multiple objects can “own” the pointer. In this case, the tile manager
owns the tiles.
Mainly, do as much as possible to reduce the time in the nested loop.
Scott
From: Interest <interest-boun...@qt-project.org> On Behalf Of Murphy, Sean
Sent: Sunday, January 30, 2022 4:59 PM
To: Tony Rietwyk <t...@rightsoft.com.au>; interest@qt-project.org
Subject: Re: [Interest] [External]Re: How to get QtConcurrent to do what I want?
Thanks for the response, but I'm not following your suggestion - or at least
I'm not seeing how it's different than what I'm doing? Maybe a little
pseudocode will help. Here's what I'm currently doing:
Tile class:
private:
QPoint mPos;
int mSize;
tile::tile(QPoint pos, int size) :
mPos(pos),
mSize(size)
{
// assigns this tile an mSize x mSize square
// from the original image starting at mPos
// pixel location in the original image
}
void tile::process()
{
// does the work on the assigned subset
}
TileManager:
private:
QVector<QSharedPointer<tile>> mTiles;
processTile(QSharedPointer<tile>& t)
{
t->process();
}
tileManager::setup(QSize tileGrid, int tileSize)
{
// generate each tile with its assignment
for(int i=0; i < tileGrid.height(); ++i)
{
for(int j=0; j < tileGrid.width(); ++j)
{
// create the new tile while assign its
// region of the original image
QSharedPointer<tile> t(new tile(
QPoint(j * tileSize, i * tileSize),
tileSize));
mTiles.append(t);
}
}
QtConcurrent::map(mTiles, processTile);
}
So I think I'm already doing what you're saying? Where I'm paying the penalty
is that the allocation of each tile is happening in one thread and I'd like to
see if I can thread out the object creation. But I don't see how to
simultaneously thread out the tile objection creation AND correctly assign the
tile its location since as far as I can tell, when QtConcurrent executes
tileManager's processTile function in parallel there's nothing I can poll
inside tileManager::processTile() that allows me to know WHICH step I'm at.
Or am I misunderstanding what you're saying?
The best thing I can come up with is that maybe I could change the type of my
mTiles vector to be a QVector<QPoint>> but then I'd still need to loop through
nested for-loop to populate all the QPoint items in the vector I want to pass
to QtConcurrent::map(). I have tried that yet to see if generating thousands of
QPoint objects is faster than generating the same number of tiles, but I can
test that out.
Sean
________________________________
From: Interest
<interest-boun...@qt-project.org<mailto:interest-boun...@qt-project.org>> on
behalf of Tony Rietwyk <t...@rightsoft.com.au<mailto:t...@rightsoft.com.au>>
Sent: Sunday, January 30, 2022 7:19 PM
To: interest@qt-project.org<mailto:interest@qt-project.org>
<interest@qt-project.org<mailto:interest@qt-project.org>>
Subject: [External]Re: [Interest] How to get QtConcurrent to do what I want?
CAUTION: This email originated from outside of the organization. Do not click
links or open attachments unless you recognize the sender and know the content
is safe.
Hi Sean,
Can you use the position of the tile as a unique key? Then the manager only
needs to calculate each tile's position in the original image. Each tile
extracts the bits, processes and notifies the result with its position.
Regards, Tony
On 31/01/2022 10:06 am, Murphy, Sean wrote:
I'm hitting a design issue with the way I'm using the QtConcurrent module to do
some image processing, and I'm wondering if someone can give some pointers?
At a high level, the software needs to do some processing on every pixel of an
image. The processing can mostly be done in parallel, so I've created the
following:
1. Tile class - responsible for doing the processing on a small subset of
the original image
* Has a constructor that takes a Position and Size. From those
parameters, the Tile knows what subset of the original image it is going to
process
* Has a process() function which will do the work on those assigned
pixels
1. TileManager class - responsible for managing the Tile objects
* Contains a for-loop that creates each Tile object, assigns it a unique
Position, and adds it to the QVector<Tile> vector
* Has a processTile(Tile& t) function which calls t.process() to tell a
given Tile to begin its work
* Calls QtConcurrent::map(tiles, processTile) to process each tile
So far this works well, but as I was timing different parts of the codebase, I
discovered that a large portion of the time is spent allocating the
QVector<Tile> vector (step 2a above) before I get to the concurrent processing
call. The reason why is obvious to me - I need to ensure that each tile is
created with a unique assignment and as far as I can see, that need to happen
in a single thread? If I could instead pass off the Tile creation to the
parallel processing step, I might be able to improve the overall performance,
but I don't see a way around it within the QtConcurrent framework.
How can I go about creating Tile objects in parallel AND ensure that each of
them gets a unique Position assignment? I could easily move the Tile allocation
into processTile(), but if I do that, I don't see a way make the unique
position assignment since I don't see how a given call to processTile() would
know where it is in the overall parallelization sequence to determine what
Position to assign to the Tile it creates. If I were using something like CUDA,
I could use things like blockIdx and threadIdx to do that, but as far as I can
see, those concepts don't exist (or at least aren't exposed) in QtConcurrent.
Any thoughts?
_______________________________________________
Interest mailing list
Interest@qt-project.org<mailto:Interest@qt-project.org>
https://lists.qt-project.org/listinfo/interest
_______________________________________________
Interest mailing list
Interest@qt-project.org
https://lists.qt-project.org/listinfo/interest
