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This change impacts only non-buffered I/O on Windows. Currently, there is a buffer per RandomAccessFile instance that is protected by a lock. The reason we maintain the buffer is non-buffered I/O requires an aligned buffer to work. XPerf traces demonstrate that we accumulate a considerable wait time while waiting for that lock. This change enables to set random access buffer size to zero which would indicate a per request allocation. We are expecting that allocation expense would be much less than I/O costs plus wait time due to the fact that the memory heap would tend to re-use page aligned allocations especially with the use of Jemalloc. This change does not affect buffer use as a read_ahead_buffer for compaction purposes.
This directory contains interfaces and implementations that isolate the rest of the package from platform details. Code in the rest of the package includes "port.h" from this directory. "port.h" in turn includes a platform specific "port_<platform>.h" file that provides the platform specific implementation. See port_posix.h for an example of what must be provided in a platform specific header file.