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fixes #326 consider nni_taskq_exec_synch()
fixes #410 kqueue implementation could be smarter
fixes #411 epoll_implementation could be smarter
fixes #426 synchronous completion can lead to panic
fixes #421 pipe close race condition/duplicate destroy
This is a major refactoring of two significant parts of the code base,
which are closely interrelated.
First the aio and taskq framework have undergone a number of simplifications,
and improvements. We have ditched a few parts of the internal API (for
example tasks no longer support cancellation) that weren't terribly useful
but added a lot of complexity, and we've made aio_schedule something that
now checks for cancellation or other "premature" completions. The
aio framework now uses the tasks more tightly, so that aio wait can
devolve into just nni_task_wait(). We did have to add a "task_prep()"
step to prevent race conditions.
Second, the entire POSIX poller framework has been simplified, and made
more robust, and more scalable. There were some fairly inherent race
conditions around the shutdown/close code, where we *thought* we were
synchronizing against the other thread, but weren't doing so adequately.
With a cleaner design, we've been able to tighten up the implementation
to remove these race conditions, while substantially reducing the chance
for lock contention, thereby improving scalability. The illumos poller
also got a performance boost by polling for multiple events.
In highly "busy" systems, we expect to see vast reductions in lock
contention, and therefore greater scalability, in addition to overall
improved reliability.
One area where we currently can do better is that there is still only
a single poller thread run. Scaling this out is a task that has to be done
differently for each poller, and carefuly to ensure that close conditions
are safe on all pollers, and that no chance for deadlock/livelock waiting
for pfd finalizers can occur.
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This replaces the epoll support with proper illumos/SunOS port
events. The port event support is structured so that it actually
is superior to epoll and kqueue, because it avoids a single master
lock on the poller. In the future we will explore this for macOS
and Linux pollers.
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This change is being made to facilitate the work done for the
kqueue port. We have created two new functions, nni_posix_pollq_init
and nni_posix_pollq_fini, which can be used when creating or destroying
the pollq nodes. Then nodes are *added* and *removed* from the pollq
structure with nni_posix_pollq_add and nni_posix_pollq_remove. The
add function in particular MUST NEVER be called unless the node has
a valid file descriptor.
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We only compile files that are appropriate for the platform. (We
still have guards in place, to allow for a future single .C file
to be built from all the sources.) We also remove the subsystem defines;
if a new platform needs to deviate from POSIX in ways beyond what we
intended here, then that platform should just copy those parts into
a new platform directory, rather than cross including portions from
POSIX.
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Apparently there are circumstances when a pipedesc may get orphaned form the
pollq. This triggers an assertion failure when it occurs. I am still
trying to understand how this can occur. Stay tuned.
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Modern Windows (Vista and later) have light weight Slim Read/Write locks
which only occupy 64 bits, and don't require any memory allocation to
create.
While here clean up a few more unreferenced variables found with the
Microsoft compilers.
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