A side effect of this flag was that we had to always use notify_all
when starting a job, instead of notify_one -- in practice, we found
experimentally that more calls to notify_one is cheaper than fewer
calls to notify_all.
The hang was caused by a subtle race. When a job is completed, its
thread must signal all the threads that might be waiting on this job.
The signaling code was attempting to signal only the minimum number
of threads -- this was important especially in the case where no threads
were waiting, then the call to notify() could be avoided.
Unfortunately, for performance reasons we're not calling notify() with
the condition lock held, this meant that between the time the number of
waiting threads was latched and the time of the notify() call, more
threads could enter their condition variable wait(), and it would
then be possible for these threads to wake up, instead of the thread
we were trying to wake up (the one waiting on the job).
It would then get stuck forever.
This bug was introduced in 2df639133b
Also add some debugging code for this kind of failure (disabled)
If hardware_concurrency() returned 2 while UTILS_HAS_HYPER_THREADING was
enabled, `mThreadCount` was resulting in zero, so jobs (such as texture
decoding) would simply never start. This problem was noticed with
GitHub Actions.
I tested this fix locally by replacing `hardware_concurrency()` with
fixed values like 1 or 2, and verified that the hang went away.
In the case where we have 2 cores, we would spawn only one thread in
the thread pool. If that thread got to try to steal() from another
thread before the main thread was adopted, it would end-up always
trying to steal from itself and enter an infinite loop.
This seems to happen during windows builds.
this is because the JobSystem's queue works as a LIFO, by creating
jobs in reverse (memory) order, we attempt to help streaming to
the d-cache on that threads -- until the point where
jobs are stolen.
we also execute the last job immediately instead of creating a job
for it -- since we're already in a job.
- parallel_for doesn't use recursion anymore to create the "leaf"
jobs, this is now done linearly on N thread (one thread per CPU).
This uses less stack space, and reduces miss-predicted branches.
- remove almost all SYSTRACE calls because they have a huge impact
on things like parallel_for() and are misleading. They can be
enabled again by setting HEAVY_SYSTRACE to true.
- we simplify the waiting code by using only a single
condition variable instead of two.
- wait() now behaves just like a looper, it will process jobs until
the one it's waiting for finishes -- before it could just sit there
(the idea was that the job would finish quickly, but that's not always
the case).
- we also make sure to never call notify_n() when it's not needed.
We track how many waiters we have and use that to decide if we need
to notify().
notify is pretty slow on all architectures, even on linux it's always
a syscall, so it's better to avoid it.
- don't use stand-alone fences, makes things ugly for no real benefit
- refactored the code a bit, hopefully it's more clear.
For jobs with that do very little work, the jobsystem can introduce
a lot of overhead, we mitigate this by:
- don't wake-up worker threads when scheduling several very small jobs,
like when scheduling the per-face jobs.
- don't wait for per-face jobs to finish -- we only did that to avoid
a copy of the job's data.
- don't use multi-threading at all if the job has too little work. We
evaluate the work using the scanline length and number of samples.
This reverts a JobSystem optimization that attempted to avoid signaling
a condition when there was no waiters. Unfortunately, there was a
race that caused the the signaling thread to miss that the waiter flag
was set, thus not signaling.
- only signal waitAndRelease() when the corresponding job finishes and
only if there is waitAndRelease() active -- instead of signaling
every time a job ends.
- don't surrender time slice when attempting to steal a job and it fails
as long as some queue has jobs.
- check that we have to wait, because taking the lock
- add a benchmark
This change more than doubles the amount of jobs we can handle per
second (~965,000 jobs/s on Pixel3)
JobSystem::waitAndRelease used to spin to wait for the job to finish,
usually this wasn't a problem because the spinning thread was
able to handle other jobs. However, in cases where no job was
available it would actually spin in burn cpu cycles.
we now use a (separate) condition variable to handle that case.
We used to only wake up a job-queue if there was already some jobs
running, the idea was that the current thread would handle the new job
as soon as calling wait(). However, there is no guarantee that wait()
will be called anytime soon.
cv.signal() is not very expensive on Android/Linux, as we're using
a custom implementation.
* Clean-up EntityManager a bit
- use tsl::robin_set instead of std::set (which should have been unordered::set
anyways).
- getListeners() now returns a vector which avoids to traverse a set twice.
Turns out that copying the set wasn't as efficient as I thought.
* Improve jobsystem a bit
We recently added a job reference counting mechanism, but we were a bit
too aggressive about taking/release references.
Also make the API more complete by adding explicit retain/release,
which is needed to allow several threads to wait on the same job.
Also improve futex code by inlining it.
* Fix a reuse after free in the job system
Jobs were destroyed and recycled while still in use
by wait() or run(). To fix this we introduce reference-counting of
jobs.
Jobs start with a ref-count of 1, which is decremented when a job
naturally finishes. Additionally, all user-facing methods acquire
a reference for the duration of the call.
* Fix an API inconsistency with JobSystem
JobSystem's API lets the user create jobs but not destroy them.
Jobs are destroyed automatically, without a way for the caller to
know when that happens.
We now explicitly enforce that jobs are no longer valid when
wait() returns. Multiple concurrent wait() are allowed however.
This is enforced by clearing the job pointer upon returning
from JobSystem::wait(Job* job).
* Rename linked-list put/get to push/pop
* Better fix for Job use after free
There was still a race condition where a run()'ed
job could be destroyed before wait() was called,
wait would then use a destroyed object.
The available APIs now are:
run() - runs and destroys a job
runAndWait() - run, then waits for and destroys a job
runAndRetain() - runs and keep a reference to the job
wait() - waits and destroys a job
wait() can only be used with a job obtained with runAndRetain().
* Get rid of unused code
This version of parallel_for has use-after-free issues anyways,
since we changed the semantics of run/wait/etc...
* Fix decRef() memory order
decRef() must ensure that all access to the
object have happened before destroying it.
* Fix memory order in atomic linked list's pop()
It needs acquire semantic, since we want to make
sure that no read/write are reordered before the
pop() -- which returns an object to the caller.
* Fix memory order on runningJobCount
we needed acquire semantic when about to destroy
the last job -- it's similar to decRef.
* Comment usages of std::memory_order_*
* Fix AtomicFreeList A-B-A bug
Turns out AtomicFreeList was not immune to the ABA bug. W're fixing
it here by using a 64-bits CAS, which is available on aarch64 and armv7.
* Add single-threaded config to Filament.
This adds a tick method to Engine and disables a couple components
in Renderer (FrameSkipper and FrameInfoManager).
This will make it easier to support WebGL, and will allow us to remove
some of the command buffer debugging stuff that we added for Vulkan.
* tick => execute, and other review feedback
* Restore the ASSERT for FFence::wait.
* JobSystem now automatically free Jobs
Until now Job allocation used a linear allocator
strategy which required to “reset” the JobSystem
periodically — typically once per frame in
filament.
This is no longer required. We use a pool allocator
now, which doesn’t add much overhead. It does
use a spin-lock for thread-safety though, since
we assume very little contention, this shouldn’t
be a problem.
* Thread Safe Object Pool Allocator
A lock-less, thread-safe object pool allocator,
now used for storing JobSystem’s jobs allocations.
This gets rid of the spin-lock introduced in the
previous cl.
