From db03cf375d602e417eda6b7a55eead91618e1398 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 9 Apr 2016 15:02:19 -0400
Subject: [PATCH] Code review/prettification for generic_xlog.c.
Improve commentary, use more specific names for the delta fields,
const-ify pointer arguments where possible, avoid assuming that
initializing only the first element of a local array will guarantee
that the remaining elements end up as we need them. (I think that
code in generic_redo actually worked, but only because InvalidBuffer
is zero; this is a particularly ugly way of depending on that ...)
---
src/backend/access/transam/generic_xlog.c | 161 +++++++++++++---------
1 file changed, 93 insertions(+), 68 deletions(-)
diff --git a/src/backend/access/transam/generic_xlog.c b/src/backend/access/transam/generic_xlog.c
index bd0a1b908df..a32f1711cf3 100644
--- a/src/backend/access/transam/generic_xlog.c
+++ b/src/backend/access/transam/generic_xlog.c
@@ -27,12 +27,12 @@
* - length of page region (OffsetNumber)
* - data - the data to place into the region ('length' number of bytes)
*
- * Unchanged regions of a page are not represented in its delta. As a
- * result, a delta can be more compact than the full page image. But having
- * an unchanged region in the middle of two fragments that is smaller than
- * the fragment header (offset and length) does not pay off in terms of the
- * overall size of the delta. For this reason, we break fragments only if
- * the unchanged region is bigger than MATCH_THRESHOLD.
+ * Unchanged regions of a page are not represented in its delta. As a result,
+ * a delta can be more compact than the full page image. But having an
+ * unchanged region between two fragments that is smaller than the fragment
+ * header (offset+length) does not pay off in terms of the overall size of
+ * the delta. For this reason, we merge adjacent fragments if the unchanged
+ * region between them is <= MATCH_THRESHOLD bytes.
*
* The worst case for delta sizes occurs when we did not find any unchanged
* region in the page. The size of the delta will be the size of the page plus
@@ -41,16 +41,16 @@
*/
#define FRAGMENT_HEADER_SIZE (2 * sizeof(OffsetNumber))
#define MATCH_THRESHOLD FRAGMENT_HEADER_SIZE
-#define MAX_DELTA_SIZE BLCKSZ + FRAGMENT_HEADER_SIZE
+#define MAX_DELTA_SIZE (BLCKSZ + FRAGMENT_HEADER_SIZE)
/* Struct of generic xlog data for single page */
typedef struct
{
Buffer buffer; /* registered buffer */
- char image[BLCKSZ]; /* copy of page image for modification */
- char data[MAX_DELTA_SIZE]; /* delta between page images */
- int dataLen; /* space consumed in data field */
bool fullImage; /* are we taking a full image of this page? */
+ int deltaLen; /* space consumed in delta field */
+ char image[BLCKSZ]; /* copy of page image for modification */
+ char delta[MAX_DELTA_SIZE]; /* delta between page images */
} PageData;
/* State of generic xlog record construction */
@@ -61,22 +61,26 @@ struct GenericXLogState
};
static void writeFragment(PageData *pageData, OffsetNumber offset,
- OffsetNumber len, Pointer data);
-static void writeDelta(PageData *pageData);
-static void applyPageRedo(Page page, Pointer data, Size dataSize);
+ OffsetNumber len, const char *data);
+static void computeDelta(PageData *pageData);
+static void applyPageRedo(Page page, const char *delta, Size deltaSize);
+
/*
- * Write next fragment into delta.
+ * Write next fragment into pageData's delta.
+ *
+ * The fragment has the given offset and length, and data points to the
+ * actual data (of length length).
*/
static void
writeFragment(PageData *pageData, OffsetNumber offset, OffsetNumber length,
- Pointer data)
+ const char *data)
{
- Pointer ptr = pageData->data + pageData->dataLen;
+ char *ptr = pageData->delta + pageData->deltaLen;
- /* Check if we have enough space */
- Assert(pageData->dataLen + sizeof(offset) +
- sizeof(length) + length <= sizeof(pageData->data));
+ /* Verify we have enough space */
+ Assert(pageData->deltaLen + sizeof(offset) +
+ sizeof(length) + length <= sizeof(pageData->delta));
/* Write fragment data */
memcpy(ptr, &offset, sizeof(offset));
@@ -86,14 +90,14 @@ writeFragment(PageData *pageData, OffsetNumber offset, OffsetNumber length,
memcpy(ptr, data, length);
ptr += length;
- pageData->dataLen = ptr - pageData->data;
+ pageData->deltaLen = ptr - pageData->delta;
}
/*
- * Make delta for given page.
+ * Compute the delta record for given page.
*/
static void
-writeDelta(PageData *pageData)
+computeDelta(PageData *pageData)
{
Page page = BufferGetPage(pageData->buffer, NULL, NULL,
BGP_NO_SNAPSHOT_TEST),
@@ -106,6 +110,8 @@ writeDelta(PageData *pageData)
imageLower = ((PageHeader) image)->pd_lower,
imageUpper = ((PageHeader) image)->pd_upper;
+ pageData->deltaLen = 0;
+
for (i = 0; i < BLCKSZ; i++)
{
bool match;
@@ -181,22 +187,22 @@ writeDelta(PageData *pageData)
char tmp[BLCKSZ];
memcpy(tmp, image, BLCKSZ);
- applyPageRedo(tmp, pageData->data, pageData->dataLen);
- if (memcmp(tmp, page, pageLower)
- || memcmp(tmp + pageUpper, page + pageUpper, BLCKSZ - pageUpper))
+ applyPageRedo(tmp, pageData->delta, pageData->deltaLen);
+ if (memcmp(tmp, page, pageLower) != 0 ||
+ memcmp(tmp + pageUpper, page + pageUpper, BLCKSZ - pageUpper) != 0)
elog(ERROR, "result of generic xlog apply does not match");
}
#endif
}
/*
- * Start new generic xlog record.
+ * Start new generic xlog record for modifications to specified relation.
*/
GenericXLogState *
GenericXLogStart(Relation relation)
{
- int i;
GenericXLogState *state;
+ int i;
state = (GenericXLogState *) palloc(sizeof(GenericXLogState));
@@ -209,24 +215,30 @@ GenericXLogStart(Relation relation)
/*
* Register new buffer for generic xlog record.
+ *
+ * Returns pointer to the page's image in the GenericXLogState, which
+ * is what the caller should modify.
+ *
+ * If the buffer is already registered, just return its existing entry.
*/
Page
GenericXLogRegister(GenericXLogState *state, Buffer buffer, bool isNew)
{
int block_id;
- /* Place new buffer to unused slot in array */
+ /* Search array for existing entry or first unused slot */
for (block_id = 0; block_id < MAX_GENERIC_XLOG_PAGES; block_id++)
{
PageData *page = &state->pages[block_id];
if (BufferIsInvalid(page->buffer))
{
+ /* Empty slot, so use it (there cannot be a match later) */
page->buffer = buffer;
- memcpy(page->image, BufferGetPage(buffer, NULL, NULL,
- BGP_NO_SNAPSHOT_TEST), BLCKSZ);
- page->dataLen = 0;
page->fullImage = isNew;
+ memcpy(page->image,
+ BufferGetPage(buffer, NULL, NULL, BGP_NO_SNAPSHOT_TEST),
+ BLCKSZ);
return (Page) page->image;
}
else if (page->buffer == buffer)
@@ -239,15 +251,16 @@ GenericXLogRegister(GenericXLogState *state, Buffer buffer, bool isNew)
}
}
- elog(ERROR, "maximum number of %d generic xlog buffers is exceeded",
+ elog(ERROR, "maximum number %d of generic xlog buffers is exceeded",
MAX_GENERIC_XLOG_PAGES);
-
/* keep compiler quiet */
return NULL;
}
/*
* Unregister particular buffer for generic xlog record.
+ *
+ * XXX this is dangerous and should go away.
*/
void
GenericXLogUnregister(GenericXLogState *state, Buffer buffer)
@@ -274,7 +287,8 @@ GenericXLogUnregister(GenericXLogState *state, Buffer buffer)
}
/*
- * Put all changes in registered buffers to generic xlog record.
+ * Apply changes represented by GenericXLogState to the actual buffers,
+ * and emit a generic xlog record.
*/
XLogRecPtr
GenericXLogFinish(GenericXLogState *state)
@@ -291,33 +305,35 @@ GenericXLogFinish(GenericXLogState *state)
for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
{
+ PageData *pageData = &state->pages[i];
+ Page page;
char tmp[BLCKSZ];
- PageData *page = &state->pages[i];
- if (BufferIsInvalid(page->buffer))
+ if (BufferIsInvalid(pageData->buffer))
continue;
- /* Swap current and saved page image. */
- memcpy(tmp, page->image, BLCKSZ);
- memcpy(page->image, BufferGetPage(page->buffer, NULL, NULL,
- BGP_NO_SNAPSHOT_TEST), BLCKSZ);
- memcpy(BufferGetPage(page->buffer, NULL, NULL,
- BGP_NO_SNAPSHOT_TEST), tmp, BLCKSZ);
+ page = BufferGetPage(pageData->buffer, NULL, NULL,
+ BGP_NO_SNAPSHOT_TEST);
- if (page->fullImage)
+ /* Swap current and saved page image. */
+ memcpy(tmp, pageData->image, BLCKSZ);
+ memcpy(pageData->image, page, BLCKSZ);
+ memcpy(page, tmp, BLCKSZ);
+
+ if (pageData->fullImage)
{
/* A full page image does not require anything special */
- XLogRegisterBuffer(i, page->buffer, REGBUF_FORCE_IMAGE);
+ XLogRegisterBuffer(i, pageData->buffer, REGBUF_FORCE_IMAGE);
}
else
{
/*
- * In normal mode, calculate delta and write it as data
+ * In normal mode, calculate delta and write it as xlog data
* associated with this page.
*/
- XLogRegisterBuffer(i, page->buffer, REGBUF_STANDARD);
- writeDelta(page);
- XLogRegisterBufData(i, page->data, page->dataLen);
+ XLogRegisterBuffer(i, pageData->buffer, REGBUF_STANDARD);
+ computeDelta(pageData);
+ XLogRegisterBufData(i, pageData->delta, pageData->deltaLen);
}
}
@@ -327,13 +343,13 @@ GenericXLogFinish(GenericXLogState *state)
/* Set LSN and mark buffers dirty */
for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
{
- PageData *page = &state->pages[i];
+ PageData *pageData = &state->pages[i];
- if (BufferIsInvalid(page->buffer))
+ if (BufferIsInvalid(pageData->buffer))
continue;
- PageSetLSN(BufferGetPage(page->buffer, NULL, NULL,
+ PageSetLSN(BufferGetPage(pageData->buffer, NULL, NULL,
BGP_NO_SNAPSHOT_TEST), lsn);
- MarkBufferDirty(page->buffer);
+ MarkBufferDirty(pageData->buffer);
}
END_CRIT_SECTION();
}
@@ -343,13 +359,15 @@ GenericXLogFinish(GenericXLogState *state)
START_CRIT_SECTION();
for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
{
- PageData *page = &state->pages[i];
+ PageData *pageData = &state->pages[i];
- if (BufferIsInvalid(page->buffer))
+ if (BufferIsInvalid(pageData->buffer))
continue;
- memcpy(BufferGetPage(page->buffer, NULL, NULL,
- BGP_NO_SNAPSHOT_TEST), page->image, BLCKSZ);
- MarkBufferDirty(page->buffer);
+ memcpy(BufferGetPage(pageData->buffer, NULL, NULL,
+ BGP_NO_SNAPSHOT_TEST),
+ pageData->image,
+ BLCKSZ);
+ MarkBufferDirty(pageData->buffer);
}
END_CRIT_SECTION();
}
@@ -360,7 +378,9 @@ GenericXLogFinish(GenericXLogState *state)
}
/*
- * Abort generic xlog record.
+ * Abort generic xlog record construction. No changes are applied to buffers.
+ *
+ * Note: caller is responsible for releasing locks/pins on buffers, if needed.
*/
void
GenericXLogAbort(GenericXLogState *state)
@@ -372,10 +392,10 @@ GenericXLogAbort(GenericXLogState *state)
* Apply delta to given page image.
*/
static void
-applyPageRedo(Page page, Pointer data, Size dataSize)
+applyPageRedo(Page page, const char *delta, Size deltaSize)
{
- Pointer ptr = data,
- end = data + dataSize;
+ const char *ptr = delta;
+ const char *end = delta + deltaSize;
while (ptr < end)
{
@@ -399,10 +419,11 @@ applyPageRedo(Page page, Pointer data, Size dataSize)
void
generic_redo(XLogReaderState *record)
{
- uint8 block_id;
- Buffer buffers[MAX_GENERIC_XLOG_PAGES] = {InvalidBuffer};
XLogRecPtr lsn = record->EndRecPtr;
+ Buffer buffers[MAX_GENERIC_XLOG_PAGES];
+ uint8 block_id;
+ /* Protect limited size of buffers[] array */
Assert(record->max_block_id < MAX_GENERIC_XLOG_PAGES);
/* Iterate over blocks */
@@ -411,20 +432,24 @@ generic_redo(XLogReaderState *record)
XLogRedoAction action;
if (!XLogRecHasBlockRef(record, block_id))
+ {
+ buffers[block_id] = InvalidBuffer;
continue;
+ }
action = XLogReadBufferForRedo(record, block_id, &buffers[block_id]);
/* Apply redo to given block if needed */
if (action == BLK_NEEDS_REDO)
{
- Pointer blockData;
- Size blockDataSize;
Page page;
+ char *blockDelta;
+ Size blockDeltaSize;
- page = BufferGetPage(buffers[block_id], NULL, NULL, BGP_NO_SNAPSHOT_TEST);
- blockData = XLogRecGetBlockData(record, block_id, &blockDataSize);
- applyPageRedo(page, blockData, blockDataSize);
+ page = BufferGetPage(buffers[block_id], NULL, NULL,
+ BGP_NO_SNAPSHOT_TEST);
+ blockDelta = XLogRecGetBlockData(record, block_id, &blockDeltaSize);
+ applyPageRedo(page, blockDelta, blockDeltaSize);
PageSetLSN(page, lsn);
MarkBufferDirty(buffers[block_id]);