RFR: 8357959: (bf) ByteBuffer.allocateDirect initialization can result in large TTSP spikes [v7]

[Aleksey Shipilev]

On Tue, 3 Jun 2025 08:35:45 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
>> B...
>
> Rohitash Kumar has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address PR Comments (rename bench and reduce max alloc size)

Now that clearing code is a bit more complicated, I think we need to extend the test that verifies that DBB is properly initialized. There is already one test, but it is unsatisfactory.

Amend it like this:  [8357959-test.txt](https://github.com/user-attachments/files/20565736/8357959-test.txt) -- then run on your new code (`make test TEST=java/nio/Buffer/AllocateDirectInit.java`).

src/java.base/share/classes/java/nio/Bits.java line 248:

> 246:      *        the starting memory address
> 247:      * @param size
> 248:      *        the number of bytes to set

Bikeshedding: This is probably `count`, not `size`.

src/java.base/share/classes/java/nio/Bits.java line 260:

> 258:             offset += len;
> 259:         }
> 260:     }

So this becomes:

Suggestion:

    static void setMemory(long srcAddr, long count, byte value) {
        long offset = 0;
        while (offset < count) {
            long len = Math.min(UNSAFE_SET_THRESHOLD, count - offset);
            UNSAFE.setMemory(srcAddr + offset, len, value);
            offset += len;
        }
    }

-------------

PR Review: https://git.openjdk.org/jdk/pull/25487#pullrequestreview-2891346767
PR Review Comment: https://git.openjdk.org/jdk/pull/25487#discussion_r2123164214
PR Review Comment: https://git.openjdk.org/jdk/pull/25487#discussion_r2123168075