More JNI write batch performance fixes

Second instance of - When passing a direct bytebuffer, slice() it first before passing over JNI
Use critical section instead of env->GetByteArrayElements to avoid a copy of a potentially very large array, only part of which we may need. It has drawbacks, but for demonstrating performance and working round the byte[] version’s performance bug,
it is invaluable.

Author: alanpaxton

java/jmh/WriteBatch.md

@@ -481,7 +481,7 @@ WriteBatchBenchmarks.putWriteBatchNative      10000000         16
 WriteBatchBenchmarks.putWriteBatchNativeBB    10000000         16                20         4096                  131072        false  thrpt    5   775264.705 ±  21106.839  ops/s
 WriteBatchBenchmarks.putWriteBatchNativeBB    10000000         16                20         4096                 1048576        false  thrpt    5   128399.860 ±   9716.046  ops/s
 
-Performance bugfix for native direct buffers:
+Performance bugfix for native direct buffers, and re-run the test above (names have changed):
 ```
 java -jar target/rocksdbjni-jmh-1.0-SNAPSHOT-benchmarks.jar WriteBatchBenchmarks.putWriteBatch -p keySize="16" -p valueSize="4096" -p keyCount="10000000" -p numOpsPerBatch="20" -p writeBatchAllocation="131072","1048576" -p writeToDB="false"
 ```
@@ -496,6 +496,24 @@ WriteBatchBenchmarks.putWriteBatchNativeD    10000000         16
 WriteBatchBenchmarks.putWriteBatchNativeI    10000000         16                20         4096                  131072        false  thrpt    5  1656172.994 ±  65982.655  ops/s
 WriteBatchBenchmarks.putWriteBatchNativeI    10000000         16                20         4096                 1048576        false  thrpt    5   608036.465 ± 148801.987  ops/s
 
+It turns out that the *indirect* version (`putWriteBatchNativeI`) uses `JNI.GetByteArrayElements()` and that does a copy in the tests I am running; I have previously seen it not do
+a copy. If we use a critical section instead (and there are difficulties in general with this, see the JNI documentation etc) we get back the performance that has gone
+missing.
+
+```
+java -jar target/rocksdbjni-jmh-1.0-SNAPSHOT-benchmarks.jar WriteBatchBenchmarks.putWriteBatch -p keySize="16" -p valueSize="4096" -p keyCount="10000000" -p numOpsPerBatch="20" -p writeBatchAllocation="131072","1048576" -p writeToDB="false"
+```
+
+Benchmark                                  (keyCount)  (keySize)  (numOpsPerBatch)  (valueSize)  (writeBatchAllocation)  (writeToDB)   Mode  Cnt        Score         Error  Units
+WriteBatchBenchmarks.putWriteBatchD          10000000         16                20         4096                  131072        false  thrpt    5  1749305.054 ±   31228.287  ops/s
+WriteBatchBenchmarks.putWriteBatchD          10000000         16                20         4096                 1048576        false  thrpt    5  1739204.577 ±   34841.845  ops/s
+WriteBatchBenchmarks.putWriteBatchI          10000000         16                20         4096                  131072        false  thrpt    5  1330461.139 ±   30573.622  ops/s
+WriteBatchBenchmarks.putWriteBatchI          10000000         16                20         4096                 1048576        false  thrpt    5  1344626.672 ±   66659.690  ops/s
+WriteBatchBenchmarks.putWriteBatchNativeD    10000000         16                20         4096                  131072        false  thrpt    5  3654634.857 ±  792083.344  ops/s
+WriteBatchBenchmarks.putWriteBatchNativeD    10000000         16                20         4096                 1048576        false  thrpt    5  2978031.325 ±  562536.419  ops/s
+WriteBatchBenchmarks.putWriteBatchNativeI    10000000         16                20         4096                  131072        false  thrpt    5  3053646.626 ±  521910.691  ops/s
+WriteBatchBenchmarks.putWriteBatchNativeI    10000000         16                20         4096                 1048576        false  thrpt    5  3930996.978 ± 2093063.739  ops/s
+
 
 
 

java/rocksjni/write_batch_java_native.cc

@@ -63,12 +63,13 @@ jlong Java_org_rocksdb_WriteBatchJavaNative_flushWriteBatchJavaNativeArray(
     wb = reinterpret_cast<ROCKSDB_NAMESPACE::WriteBatchJavaNative*>(jwb_handle);
   }
 
-  jbyte* buf = env->GetByteArrayElements(jbuf, nullptr);
+  jboolean is_copy;
+  jbyte* buf = reinterpret_cast<jbyte *>(env->GetPrimitiveArrayCritical(jbuf, &is_copy));
   if (env->ExceptionCheck()) {
     // exception thrown: OutOfMemoryError
     return -1L;
   }
-
+  
   try {
     wb->Append(
         ROCKSDB_NAMESPACE::Slice(reinterpret_cast<char*>(buf), jbuf_len));
@@ -77,7 +78,7 @@ jlong Java_org_rocksdb_WriteBatchJavaNative_flushWriteBatchJavaNativeArray(
     return e.Code();
   }
 
-  env->ReleaseByteArrayElements(jbuf, buf, JNI_ABORT);
+  env->ReleasePrimitiveArrayCritical(jbuf, buf, JNI_ABORT);
 
   return GET_CPLUSPLUS_POINTER(wb);
 }
@@ -140,12 +141,13 @@ jlong Java_org_rocksdb_WriteBatchJavaNative_writeWriteBatchJavaNativeArray(
     wb = reinterpret_cast<ROCKSDB_NAMESPACE::WriteBatchJavaNative*>(jwb_handle);
   }
 
-  jbyte* buf = env->GetByteArrayElements(jbuf, nullptr);
+  jboolean is_copy;
+  jbyte* buf = reinterpret_cast<jbyte *>(env->GetPrimitiveArrayCritical(jbuf, &is_copy));
   if (env->ExceptionCheck()) {
     // exception thrown: OutOfMemoryError
     return -1L;
   }
-
+  
   try {
     wb->Append(
         ROCKSDB_NAMESPACE::Slice(reinterpret_cast<char*>(buf), jbuf_len));
@@ -154,7 +156,7 @@ jlong Java_org_rocksdb_WriteBatchJavaNative_writeWriteBatchJavaNativeArray(
     return e.Code();
   }
 
-  env->ReleaseByteArrayElements(jbuf, buf, JNI_ABORT);
+  env->ReleasePrimitiveArrayCritical(jbuf, buf, JNI_ABORT);
 
   ROCKSDB_NAMESPACE::Status s = db->Write(*write_options, wb);
 

java/src/main/java/org/rocksdb/WriteBatchJavaNative.java

@@ -221,10 +221,11 @@ void write(final RocksDB db, final WriteOptions writeOptions) throws RocksDBExce
 
     buffer.flip();
     if (buffer.isDirect()) {
+      ByteBuffer slice = buffer.slice();
       setNativeHandle(
           writeWriteBatchJavaNativeDirect(db.getNativeHandle(), writeOptions.getNativeHandle(),
               // assert position == 0 (we just flipped)
-              getNativeHandle(), buffer.capacity(), buffer, buffer.position(), buffer.limit()));
+              getNativeHandle(), buffer.capacity(), slice, 0, slice.limit()));
     } else {
       setNativeHandle(
           writeWriteBatchJavaNativeArray(db.getNativeHandle(), writeOptions.getNativeHandle(),