optimized index snapshot reading/writing (#561)

the previous implementation of in-memory index snapshots serialise data to memory before writing to disk and vice versa when reading. This leads to some memory spikes which end up pushing useful data out of system cache and also cause stalls on I/O operations.

this change moves more code to a streaming serialisation approach which scales better from a memory usage perspective and also performs much better

1 files changed, 84 insertions, 10 deletions

diff --git a/src/zenutil/basicfile.cpp b/src/zenutil/basicfile.cpp
index 1dce71e60..7d0edaa5d 100644
--- a/src/zenutil/basicfile.cpp
+++ b/src/zenutil/basicfile.cpp
@@ -76,16 +76,15 @@ BasicFile::Open(const std::filesystem::path& FileName, Mode InMode, std::error_c
 
 	const DWORD dwShareMode = FILE_SHARE_READ | (EnumHasAllFlags(InMode, Mode::kPreventWrite) ? 0 : FILE_SHARE_WRITE) |
 							  (EnumHasAllFlags(InMode, Mode::kPreventDelete) ? 0 : FILE_SHARE_DELETE);
-	const DWORD dwFlagsAndAttributes =
-		FILE_ATTRIBUTE_NORMAL | (EnumHasAllFlags(InMode, Mode::kDeleteOnClose) ? FILE_FLAG_DELETE_ON_CLOSE : 0);
-	const HANDLE hTemplateFile = nullptr;
-	const HANDLE FileHandle	   = CreateFile(FileName.c_str(),
-											dwDesiredAccess,
-											dwShareMode,
-											/* lpSecurityAttributes */ nullptr,
-											dwCreationDisposition,
-											dwFlagsAndAttributes,
-											hTemplateFile);
+	const DWORD	 dwFlagsAndAttributes = FILE_ATTRIBUTE_NORMAL;
+	const HANDLE hTemplateFile		  = nullptr;
+	const HANDLE FileHandle			  = CreateFile(FileName.c_str(),
+										   dwDesiredAccess,
+										   dwShareMode,
+										   /* lpSecurityAttributes */ nullptr,
+										   dwCreationDisposition,
+										   dwFlagsAndAttributes,
+										   hTemplateFile);
 
 	if (FileHandle == INVALID_HANDLE_VALUE)
 	{
@@ -588,6 +587,8 @@ LockFile::Update(CbObject Payload, std::error_code& Ec)
 	BasicFile::Write(Payload.GetBuffer(), 0, Ec);
 }
 
+//////////////////////////////////////////////////////////////////////////
+
 BasicFileBuffer::BasicFileBuffer(BasicFile& Base, uint64_t BufferSize)
 : m_Base(Base)
 , m_Buffer(nullptr)
@@ -662,6 +663,79 @@ BasicFileBuffer::MakeView(uint64_t Size, uint64_t FileOffset)
 	return MemoryView(m_Buffer + (FileOffset - m_BufferStart), Size);
 }
 
+//////////////////////////////////////////////////////////////////////////
+
+BasicFileWriter::BasicFileWriter(BasicFile& Base, uint64_t BufferSize)
+: m_Base(Base)
+, m_Buffer(nullptr)
+, m_BufferSize(BufferSize)
+, m_BufferStart(0)
+, m_BufferEnd(0)
+{
+	m_Buffer = (uint8_t*)Memory::Alloc(m_BufferSize);
+}
+
+BasicFileWriter::~BasicFileWriter()
+{
+	Flush();
+	Memory::Free(m_Buffer);
+}
+
+void
+BasicFileWriter::Write(void* Data, uint64_t Size, uint64_t FileOffset)
+{
+	if (m_Buffer == nullptr || (Size >= m_BufferSize))
+	{
+		m_Base.Write(Data, Size, FileOffset);
+		return;
+	}
+
+	// Note that this only supports buffering of sequential writes!
+
+	if (FileOffset != m_BufferEnd)
+	{
+		Flush();
+		m_BufferStart = m_BufferEnd = FileOffset;
+	}
+
+	while (Size)
+	{
+		const uint64_t RemainingBufferCapacity = m_BufferStart + m_BufferSize - m_BufferEnd;
+		const uint64_t BlockWriteBytes		   = Min(RemainingBufferCapacity, Size);
+		const uint64_t BufferWriteOffset	   = FileOffset - m_BufferStart;
+
+		ZEN_ASSERT_SLOW(BufferWriteOffset < m_BufferSize);
+		ZEN_ASSERT_SLOW((BufferWriteOffset + BlockWriteBytes) <= m_BufferSize);
+
+		memcpy(m_Buffer + BufferWriteOffset, Data, BlockWriteBytes);
+
+		Size -= BlockWriteBytes;
+		m_BufferEnd += BlockWriteBytes;
+		FileOffset += BlockWriteBytes;
+
+		if ((m_BufferEnd - m_BufferStart) == m_BufferSize)
+		{
+			Flush();
+		}
+	}
+}
+
+void
+BasicFileWriter::Flush()
+{
+	const uint64_t BufferedBytes = m_BufferEnd - m_BufferStart;
+
+	if (BufferedBytes == 0)
+		return;
+
+	const uint64_t WriteOffset = m_BufferStart;
+	m_BufferStart			   = m_BufferEnd;
+
+	m_Base.Write(m_Buffer, BufferedBytes, WriteOffset);
+}
+
+//////////////////////////////////////////////////////////////////////////
+
 /*
 		___________              __
 		\__    ___/___   _______/  |_  ______