gh-133136: Limit excess memory held by QSBR #135107
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The free threading build uses QSBR to delay the freeing of dictionary keys and list arrays when the objects are accessed by multiple threads in order to allow concurrent reads to proceeed with holding the object lock. The requests are processed in batches to reduce execution overhead, but for large memory blocks this can lead to excess memory usage. Take into account the size of the memory block when deciding when to process QSBR requests.
corona10
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Jun 7, 2025
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| should_advance_qsbr(_PyThreadStateImpl *tstate, size_t size) | ||
| { | ||
| // If the deferred memory exceeds 1 MiB, we force an advance in the | ||
| // shared QSBR sequence number to limit excess memory usage. | ||
| static const size_t QSBR_DEFERRED_LIMIT = 1024 * 1024; | ||
| if (size > QSBR_DEFERRED_LIMIT) { | ||
| tstate->qsbr->memory_deferred = 0; | ||
| return 1; | ||
| } | ||
|
|
||
| tstate->qsbr->memory_deferred += size; | ||
| if (tstate->qsbr->memory_deferred > QSBR_DEFERRED_LIMIT) { | ||
| tstate->qsbr->memory_deferred = 0; | ||
| return 1; | ||
| } | ||
| return 0; | ||
| } |
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Do we need early return here?
It looks like it will be same eventually.
Suggested change
| should_advance_qsbr(_PyThreadStateImpl *tstate, size_t size) | |
| { | |
| // If the deferred memory exceeds 1 MiB, we force an advance in the | |
| // shared QSBR sequence number to limit excess memory usage. | |
| static const size_t QSBR_DEFERRED_LIMIT = 1024 * 1024; | |
| if (size > QSBR_DEFERRED_LIMIT) { | |
| tstate->qsbr->memory_deferred = 0; | |
| return 1; | |
| } | |
| tstate->qsbr->memory_deferred += size; | |
| if (tstate->qsbr->memory_deferred > QSBR_DEFERRED_LIMIT) { | |
| tstate->qsbr->memory_deferred = 0; | |
| return 1; | |
| } | |
| return 0; | |
| } | |
| should_advance_qsbr(_PyThreadStateImpl *tstate, size_t size) | |
| { | |
| // If the deferred memory exceeds 1 MiB, we force an advance in the | |
| // shared QSBR sequence number to limit excess memory usage. | |
| static const size_t QSBR_DEFERRED_LIMIT = 1024 * 1024; | |
| tstate->qsbr->memory_deferred += size; | |
| if (tstate->qsbr->memory_deferred > QSBR_DEFERRED_LIMIT) { | |
| tstate->qsbr->memory_deferred = 0; | |
| return 1; | |
| } | |
| return 0; | |
| } |
|
Benchmarking script, based on issue. The reported numbers are the RSS (resident-set-size) of the process on 5 second intervals, in MB.
The last item shows about as good as we can expect to do by making the QSBR processing more aggressive at freeing. |
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The free threading build uses QSBR to delay the freeing of dictionary keys and list arrays when the objects are accessed by multiple threads in order to allow concurrent reads to proceeed with holding the object lock. The requests are processed in batches to reduce execution overhead, but for large memory blocks this can lead to excess memory usage.
Take into account the size of the memory block when deciding when to process QSBR requests.