perf(cs): make inputBuffer size progressive

The current implementation of the WriteHighLevelOp class always asks
for maxBlocksPerHddWriteJob_ blocks for its next inputBuffer_, i.e the
structure to land the upcoming data blocks to be written to the drive.
This can became a significant waste of memory when the write high level
operation only writes 1 block, but asks for 32, if the
MAX_BLOCKS_PER_HDD_WRITE_JOB is set to that number, for instance.

The change proposed targets limiting the use of memory for any number
of blocks written in a single write high level operation to the double
of the strictly necessary memory. Considering the previous case, the
write high level operation will ask for 1, 2, 4, 8, 16, 32 and continue
asking for input buffers of 32 blocks.

The main downside is a possible increase in the  number of drive
operations when the number of blocks written is between 2 and 15.

Signed-off-by: Dave <dave@leil.io>

Author: dmga44

src/chunkserver/chunk_high_level_ops.cc

@@ -466,8 +466,10 @@ void WriteHighLevelOp::prepareInputBufferForWrite(bool isForward) {
 	if (inputBuffer_ == nullptr) {
 		inputBuffer_ = getWriteInputBufferPool().get(
 		    isForward ? kSauWriteDataPrefixSizeForward : kSauWriteDataPrefixSize,
-		    maxBlocksPerHddWriteJob_);
+		    nextInputBufferBlockCount_);
 	}
+	nextInputBufferBlockCount_ =
+	    std::min<uint16_t>(nextInputBufferBlockCount_ * 2, maxBlocksPerHddWriteJob_);
 
 	inputBuffer_->addNewWriteOperation();
 }

src/chunkserver/chunk_high_level_ops.h

@@ -300,7 +300,10 @@ class WriteHighLevelOp : public HighLevelOp {
 	/// Prepares the input buffer for a write operation.
 	void prepareInputBufferForWrite(bool isForward);
 
-	///< Number of blocks to write to the device in one write job.
+	/// Size in blocks of the next input buffer.
+	uint16_t nextInputBufferBlockCount_ = 1;
+
+	/// Number of blocks to write to the device in one write job.
 	uint16_t maxBlocksPerHddWriteJob_;
 
 	/// Indicates if the chunk is locked for writing. If true, master will be waiting for the lock