DiscIO: Decrease RAM usage during zstd compression

By calling ZSTD_CCtx_setPledgedSrcSize, we can let zstd know
how large a chunk is going to be before which start compressing
it, which lets zstd avoid allocating more memory than needed
for various internal buffers. This greatly reduces the RAM usage
when using a high compression level with a small chunk size,
and doesn't have much of an effect in other circumstances.

A side effect of calling ZSTD_CCtx_setPledgedSrcSize is that
zstd by default will write the uncompressed size into the
compressed data stream as metadata. In order to save space,
and since the decompressed size can be figured out through
the structure of the RVZ format anyway, we disable writing
the uncompressed size by setting ZSTD_c_contentSizeFlag to 0.

1 files changed, 18 insertions, 6 deletions

diff --git a/Source/Core/DiscIO/WIACompression.cpp b/Source/Core/DiscIO/WIACompression.cpp
index 20d19c4877..4fdf6cd429 100644
--- a/Source/Core/DiscIO/WIACompression.cpp
+++ b/Source/Core/DiscIO/WIACompression.cpp
@@ -446,7 +446,7 @@ PurgeCompressor::PurgeCompressor()
 
 PurgeCompressor::~PurgeCompressor() = default;
 
-bool PurgeCompressor::Start()
+bool PurgeCompressor::Start(std::optional<u64> size)
 {
   m_buffer.clear();
   m_bytes_written = 0;
@@ -550,7 +550,7 @@ Bzip2Compressor::~Bzip2Compressor()
   BZ2_bzCompressEnd(&m_stream);
 }
 
-bool Bzip2Compressor::Start()
+bool Bzip2Compressor::Start(std::optional<u64> size)
 {
   ASSERT_MSG(DISCIO, m_stream.state == nullptr,
              "Called Bzip2Compressor::Start() twice without calling Bzip2Compressor::End()");
@@ -674,7 +674,7 @@ LZMACompressor::~LZMACompressor()
   lzma_end(&m_stream);
 }
 
-bool LZMACompressor::Start()
+bool LZMACompressor::Start(std::optional<u64> size)
 {
   if (m_initialization_failed)
     return false;
@@ -745,8 +745,11 @@ ZstdCompressor::ZstdCompressor(int compression_level)
 {
   m_stream = ZSTD_createCStream();
 
-  if (ZSTD_isError(ZSTD_CCtx_setParameter(m_stream, ZSTD_c_compressionLevel, compression_level)))
+  if (ZSTD_isError(ZSTD_CCtx_setParameter(m_stream, ZSTD_c_compressionLevel, compression_level)) ||
+      ZSTD_isError(ZSTD_CCtx_setParameter(m_stream, ZSTD_c_contentSizeFlag, 0)))
+  {
     m_stream = nullptr;
+  }
 }
 
 ZstdCompressor::~ZstdCompressor()
@@ -754,7 +757,7 @@ ZstdCompressor::~ZstdCompressor()
   ZSTD_freeCStream(m_stream);
 }
 
-bool ZstdCompressor::Start()
+bool ZstdCompressor::Start(std::optional<u64> size)
 {
   if (!m_stream)
     return false;
@@ -762,7 +765,16 @@ bool ZstdCompressor::Start()
   m_buffer.clear();
   m_out_buffer = {};
 
-  return !ZSTD_isError(ZSTD_CCtx_reset(m_stream, ZSTD_reset_session_only));
+  if (ZSTD_isError(ZSTD_CCtx_reset(m_stream, ZSTD_reset_session_only)))
+    return false;
+
+  if (size)
+  {
+    if (ZSTD_isError(ZSTD_CCtx_setPledgedSrcSize(m_stream, *size)))
+      return false;
+  }
+
+  return true;
 }
 
 bool ZstdCompressor::Compress(const u8* data, size_t size)