50 lines
2.2 KiB
Markdown
50 lines
2.2 KiB
Markdown
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20-11-2020
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# Memory Allocation In vdbesort.c
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Memory allocation is slightly different depending on:
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* whether or not SQLITE_CONFIG_SMALL_MALLOC is set, and
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* whether or not worker threads are enabled.
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## SQLITE_CONFIG_SMALL_MALLOC=0
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Assuming SQLITE_CONFIG_SMALL_MALLOC is not set, keys passed to the sorter are
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added to an in-memory buffer. This buffer is grown using sqlite3Realloc() as
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required it reaches the size configured for the main pager cache using "PRAGMA
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cache_size". i.e. if the user has executed "PRAGMA main.cache_size = -2048",
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then this buffer is allowed to grow up to 2MB in size.
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Once the buffer has grown to its threshold, keys are sorted and written to
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a temp file. If worker threads are not enabled, this is the only significant
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allocation the sorter module makes. After keys are sorted and flushed out to
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the temp file, the buffer is reused to accumulate the next batch of keys.
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If worker threads are available, then the buffer is passed to a worker thread
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to sort and flush once it is full, and a new buffer allocated to allow the
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main thread to continue to accumulate keys. Buffers are reused once they
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have been flushed, so in this case at most (nWorker+1) buffers are allocated
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and used, where nWorker is the number of configured worker threads.
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There are no other significant users of heap memory in the sorter module.
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Once sorted buffers of keys have been flushed to disk, they are read back
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either by mapping the file (via sqlite3_file.xFetch()) or else read back
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in one page at a time.
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All buffers are allocated by the main thread. A sorter object is associated
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with a single database connection, to which it holds a pointer.
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## SQLITE_CONFIG_SMALL_MALLOC=1
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This case is similar to the above, except that instead of accumulating
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multiple keys in a single large buffer, sqlite3VdbeSorterWrite() stores
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keys in a regular heap-memory linked list (one allocation per element).
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List elements are freed as they are flushed to disk, either by the main
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thread or by a worker thread.
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Each time a key is added the sorter (and an allocation made),
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sqlite3HeapNearlyFull() is called. If it returns true, the current
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list of keys is flushed to a temporary file, even if it has not yet
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reached the size threshold.
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