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Freeing memory from a finalizer waits for the GPU

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
julia
Ambito
backend, performance

Direzione di ricerca

Start with the finalizer and free policy in src/pool.jl, then read the GC workaround in test/execution.jl ("freeing memory may wait for the stream on some stacks"). Compare DEFER_FREE behavior on the rolling stack with the LTS stack's policy limitations described in the issue. Done means finalizer cleanup avoids the reported GPU wait and deadlock on each supported stack, with tests covering memory reclamation.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

performance

Freeing memory from a finalizer waits for all GPU work in the process. The finalizer of an array that isn't sub-allocated (device memory over 1 MiB, and all shared and host memory) frees it with zeMemFreeExt and the BLOCKING_FREE policy (src/pool.jl). Our kernels declare indirect access, so NEO considers every allocation in use by every kernel, and the free waits until everything submitted so far has executed, not only the work using that buffer.

That stalls whatever thread happens to collect garbage, and deadlocks when the GPU work waits for a task on that thread. The test suite already works around it by disabling GC while a kernel waits on the host (test/execution.jl, "freeing memory may wait for the stream on some stacks").

Measured on an Iris Xe (NEO 26.35), with a ~1 s kernel running on another task's stream, and the release triggered from a finalizer on thread 1 (medians of 7):

release blocks the collecting thread launch on another thread
device array, 4 KiB (sub-allocated) 0.02 ms 0.15 ms
device array, 64 MiB 863 ms 0.15 ms
shared array, 4 KiB / 64 MiB 861 / 861 ms 0.14 / 0.16 ms
host array, 4 KiB / 64 MiB 861 / 865 ms 0.14 / 0.16 ms
zeMemFree without a policy 857 ms 0.17 ms
zeMemFreeExt with DEFER_FREE 0.03 ms 0.15 ms
busy ZeImmediateCommandList 902 ms –
modules, kernels, queues, lists, events, fences ≤ 0.1 ms –

With the stream waiting on an event that a task on thread 1 signals, collecting a host array (3/3), a 64 MiB shared array (3/3) or a 64 MiB device array (2/3) on that thread hangs until a watchdog signals the event.

Unlike with CUDA (JuliaGPU/CUDA.jl#3318), launches on other threads aren't held up, and only memory frees and the busy immediate-list drain wait. So rather than deferring every release, it should be enough to free with DEFER_FREE on the rolling stack. NEO keeps deferred allocations on a list that it only processes on a later free, so reclaiming memory needs to trigger one. The LTS stack reportedly doesn't honor the free policies (see the comment in src/pool.jl), so there the blocking free, and the unbounded synchronize_all_streams before it, would have to move out of the finalizer instead.

Lingua principale
Julia
Stelle
218
Fork
36
Merge medio
1g 4h
PR unite (30g)
55

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