This is not a scheduler in the runtime, instead every goroutine is
mapped to a single OS thread - meaning 1:1 scheduling.
While this may not perform well (or at all) for large numbers of
threads, it greatly simplifies many things in the runtime. For example,
blocking syscalls can be called directly instead of having to use epoll
or similar. Also, we don't need to do anything special to call C code -
the default stack is all we need.
Using a global lock may be slow, but it is certainly simple and safe.
If this global lock becomes a bottleneck, we can of course look into
making the GC truly support multithreading.
This adds support for the well-known Boehm GC. It's significantly faster
than our own naive GC and could be used as an alternative on bigger
systems.
In the future, this GC might also be supported on WebAssembly with some
extra work. Right now it's Linux only (though Windows/MacOS shouldn't be
too difficult to add).
For example, with -gc=none and -gc=leaking, no heap needs to be
allocated when initializing the runtime. And some GCs (like -gc=custom)
are responsible for allocating the heap themselves.
This moves all scheduler code into a separate file that is only compiled
when there's a scheduler in use (the tasks or asyncify scheduler, which
are both cooperative). The main goal of this change is to make it easier
to add a new "scheduler" based on OS threads.
It also fixes a few subtle issues with `-gc=none`:
- Gosched() panicked. This is now fixed to just return immediately
(the only logical thing to do when there's only one goroutine).
- Timers aren't supported without a scheduler, but the relevant code
was still present and would happily add a timer to the queue. It
just never ran. So now it exits with a runtime error, similar to any
blocking operation.
This is similar to https://github.com/tinygo-org/tinygo/pull/3899, but
smaller and hopefully just as efficient.
Thanks to @HattoriHanzo031 for starting this work, benchmarking, and for
improving the performance of the code even further.
Every time we overflow the stack, we have to do a full rescan of the heap. Making this larger
means fewer overflows and thus fewer secondary+ heap scans.
Instead of markGlobals calling markRoots unconditionally (which doesn't
make sense for -gc=none and -gc=leaking), provide markRoots as a
callback function.
This is in preparation for -gc=boehm, where the previous design is even
more awkward and a callback makes far more sense.
I've tested the size impact using `make smoketest XTENSA=0`. There is
none, except for two cases:
* One with `-opt=0` so const-propagation for the callback didn't take
place.
* One other on AVR, I don't know why but as it's only 16 bytes in a
very specific case I'm going to assume it's just a random change in
compiler output that caused a size difference.
This is not very useful in itself, but makes it possible to detect this
address in the output. See the next commit.
This adds around 50 bytes to each binary (except for AVR and wasm). This
is unfortunate, but I think this feature is quite useful still.
A future enhancement might be to create a build tag for extended panic
information that's not set by default.
We have an optimization for this specific pattern, but it's really just
a hack. With the addition of unsafe.Add in Go 1.17 we can directly
specify the intent instead and eventually remove this special case.
The code is also easier to read.
This implements the block-based GC as a partially precise GC. This means
that for most heap allocations it is known which words contain a pointer
and which don't. This should in theory make the GC faster (because it
can skip non-pointer object) and have fewer false positives in a GC
cycle. It does however use a bit more RAM to store the layout of each
object.
Right now this GC seems to be slower than the conservative GC, but
should be less likely to run out of memory as a result of false
positives.
Most of the code of the conservative GC can be reused for the precise
GC. So before adding precise GC support, this commit just moves code
around to make the next commit cleaner. It is a non-functional change.
Ayke van Laethem
Damian Gryski
dkegel-fastly
Anuraag Agrawal