Increasingly in the past year I've been writing GC-aware code. On the one
hand, my machine learning related stuff has sped up quite a fair bit. On
the other hand additional profiling shows me how far I've yet to go. I've
been thinking of moving a bunch of stuff off-heap. But at the same time,
having come from a history of writing shitty C code, I know I need GC.
So my question is this - given a structure (an abridged version of the
Dense tensor in Gorgonia) like this:
type Dense struct {
metadata *AP // Go allocated memory
data unsafe.Pointer // off heap or otherwise inaccessible via Go
l, c int // Go allocated memory
}
What happens when the GC scanner scans the data field? This trick was
created to work with heterogenous memory (CUDA/GPU memory), and comes with
a bag of flags to indicate if the memory is accessible or not. The thing
about using CUDA is that the cycles are shortlived. The pattern goes like
this: acquire, do operation, deallocate memory, invalidate all pointers,
chuck the dense tensor into an object pool. This cycle has only been
observed to happen between GC scan passes with CUDA memory.
As such I have no idea what would happen if the GC scanner hits a
unsafe.Pointer that it cannot access. Will the pointer be marked as
unaccessible? Does it panic with SIGBUS? What are the long term
implications of this, if say a separate memory pool were to be allocated
outside.
Some considerations: there are typically 200k of these *Dense tensors
around in a small neural network that needs to be kept alive. It'd be
preferable to not have the entire struct be allocated offheap, because
that's definitely a sign of trouble to come. I'm aware that I can
preallocate an arena of []bytes, and use it, but I'm currently weighting my
options.
Some advice would be quite nice. Please feel free to tell me I'm headed the
wrong direction
Thank you
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