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Remove unnecessary memcpy when reading WAL record fitting on page

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When reading a WAL record, its contents are copied into an intermediate
buffer.  However, doing so is not necessary if the record fits fully
into the current page, saving one memcpy for each such record.  The
allocation handling of the intermediate buffer is also now done only
when a record crosses a page boundary, shaving some extra cycles when
reading a WAL record.

Author: Andrey Lepikhov
Reviewed-by: Kyotaro Horiguchi, Heikki Linnakangas
Discussion: https://postgr.es/m/c2ea54dd-a1d3-80eb-ddbf-7e6f258e615e@postgrespro.ru
This commit is contained in:
Michael Paquier 2018-11-19 10:25:48 +09:00
parent 79376e0712
commit 285bd0ac4a
2 changed files with 13 additions and 17 deletions
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3
src/backend/access/transam/xlog.c
View File

@ -4203,9 +4203,6 @@ CleanupBackupHistory(void)
* If no valid record is available, returns NULL, or fails if emode is PANIC.
* (emode must be either PANIC, LOG). In standby mode, retries until a valid
* record is available.
*
* The record is copied into readRecordBuf, so that on successful return,
* the returned record pointer always points there.
*/
static XLogRecord *
ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,

19
src/backend/access/transam/xlogreader.c
View File

@ -353,6 +353,15 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
gotheader = false;
}
len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
if (total_len > len)
{
/* Need to reassemble record */
char *contdata;
XLogPageHeader pageHeader;
char *buffer;
uint32 gotlen;
/*
* Enlarge readRecordBuf as needed.
*/
@ -366,15 +375,6 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
goto err;
}
len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
if (total_len > len)
{
/* Need to reassemble record */
char *contdata;
XLogPageHeader pageHeader;
char *buffer;
uint32 gotlen;
/* Copy the first fragment of the record from the first page. */
memcpy(state->readRecordBuf,
state->readBuf + RecPtr % XLOG_BLCKSZ, len);
@ -479,7 +479,6 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
state->EndRecPtr = RecPtr + MAXALIGN(total_len);
state->ReadRecPtr = RecPtr;
memcpy(state->readRecordBuf, record, total_len);
}
/*