I have a big array (like a few GB) which is operated upon by some functions.
As these functions act purely local, an obvious idea is:
- (void)someFunction
{
nbrOfThreads = ...
sizeOfBigArray = ... a few GB
stride = sizeOfBigArray / nbrOfThreads
dispatch_apply( nbrOfThreads, queue, ^void(size_t idx)
{
start = idx * stride
end = start + stride
index = start
while ( index < end )
{
mask = ...
bigArray[index] |= mask
index += … something positive…
}
}
)
}
As my computer has just 8 CPUs, I thought that using nbrOfThreads > 8 would be
silly: adding overhead without gaining anything.
Turns out this is quite wrong. One function (called threadHappyFunction) works
more than 10 times faster using nbrOfThreads = a few ten-thousand (as compared
to nbrOfThreads = 8).
This is nice, but I would like to know why can this happen.
Another function does not like threads at all:
- (void)threadShyFunction
{
nbrOfThreads = ...
uint64_t *bigArrayAsLongs = (uint64_t *)bigArray
sizeOfBigArrayInLongs = ...
stride = sizeOfBigArrayInLongs / nbrOfThreads
uint64_t *template = ...
sizeOfTemplate = not more than a few dozen longs
dispatch_apply( nbrOfThreads, queue, ^void(size_t idx)
{
start = idx * stride
end = start + stride
offset = start
while ( offset + sizeOfTemplate < end )
{
for ( i = 0 ..< sizeOfTemplate )
bigArrayAsLongs[offset + i] |= template[i]
offset += sizeOfTemplate
}
}
)
}
This works, but for nbrOfThreads > 1 it gets slower instead of faster. Up to
hundred times slower for moderately big nbrOfThreads.
This really bothers me.
Why are the threads seemingly blocking each other?
What is going on here?
How can I investigate this?
Gerriet.
P.S. macOS 12, Xcode 8, ObjC or Swift.
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