Integrated: 8357959: (bf) ByteBuffer.allocateDirect initialization can result in large TTSP spikes
Rohitash Kumar
duke at openjdk.org
Thu Jun 12 12:26:43 UTC 2025
On Wed, 28 May 2025 09:30:56 GMT, Rohitash Kumar <duke at openjdk.org> wrote:
> ByteBuffer.allocateDirect uses UNSAFE.setMemory, causing high time-to-safepoint (100+ ms) for large (100 MB+) allocations.
>
> This PR applies a simple fix by chunking the zeroing operation within ByteBuffers. A more robust solution would be to add chunking inside UNSAFE.setMemory itself. However Its not that straightforward as mentioned by Aleksey in [JDK-8357959](https://bugs.openjdk.org/browse/JDK-8357959)
>>Looks like all current uses we care about are in Buffers. Taking a safepoint within cleaning would open some questions whether any VM code expect to see semi-initialized area we are busy cleaning up. For Buffers, this question does not arise. Therefore, we can do the fix in Buffers first, without changing the Unsafe itself.
>
> I can pursue that if its preferred. I chose 1 MB as a chunk size some what arbitrarily I am open to suggestion, if there are better options.
>
> For verification, I tested the fix against the reproducer - [gist](https://gist.github.com/rk-kmr/be4322b72a14ae04aeefc0260c01acf6) and confirmed that ttsp timing were lower.
>
> **before**
>
> 0.444s][info][safepoint,stats] ThreadDump [ 13 1 ][ 194156625 65291 194221916 ] 0
> [0.662s][info][safepoint,stats] ThreadDump [ 13 1 ][ 200013875 87834 200101709 ] 0
> [0.858s][info][safepoint,stats] ThreadDump [ 13 1 ][ 183762583 43417 183806000 ] 0
> [1
>
> **after**
>
> 1.705s][info][safepoint,stats] ThreadDump [ 11 1 ][ 92792 24958 117750 ] 0
> [1.724s][info][safepoint,stats] ThreadDump [ 11 1 ][ 497375 94041 591416 ] 0
> [1.736s][info][safepoint,stats] ThreadDump [ 11 1 ][ 156750 47208 203958 ] 0
> [1.747s][info][safepoint,stats] ThreadDump [ 11 1 ][ 121958 28334 150292 ] 0
>
>
> I added a benchmark to ensure that chunking doesn't introduce significant overhead across different allocation sizes, and following results confirm that.
>
> **Before**
>
> Benchmark (bytes) Mode Cnt Score Error Units
> ByteBufferAllocationBenchmark.allocateDirectBuffer 1...
This pull request has now been integrated.
Changeset: e5ce5c57
Author: Rohitash Kumar <kurohita at amazon.com>
Committer: Aleksey Shipilev <shade at openjdk.org>
URL: https://git.openjdk.org/jdk/commit/e5ce5c57c83972ff52758a804c942986cab74ca7
Stats: 132 lines in 4 files changed: 115 ins; 0 del; 17 mod
8357959: (bf) ByteBuffer.allocateDirect initialization can result in large TTSP spikes
Reviewed-by: shade, alanb
-------------
PR: https://git.openjdk.org/jdk/pull/25487
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