DiscIO: Decrease RAM usage during zstd compression #9137
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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.
leoetlino
approved these changes
Oct 14, 2020
| @@ -1564,7 +1564,7 @@ WIARVZFileReader<RVZ>::ProcessAndCompress(CompressThreadState* state, CompressPa | |||
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| if (state->compressor) | |||
| { | |||
| if (!state->compressor->Start()) | |||
| if (!state->compressor->Start(entry.exception_lists.size() + entry.main_data.size())) | |||
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I had to check that exceptions_list indeed stored bytes rather than exception structs (as the name would imply IMO). That's probably just because I'm unfamiliar with this code, though :)
Still, it might be worth renaming this member to exception_list_data for more clarity and for consistency with main_data in a followup PR?
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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_setPledgedSrcSizeis 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 settingZSTD_c_contentSizeFlagto 0.