The attached patch makes O_DIRECT work on Linux in BerkeleyDB 4.5.20. (You
will need to manually define LINUX_NEEDS_PAGE_ALIGNMENT if you're using a
kernel older than 2.6.)
The main reason to use this patch is to conserve memory - ordinarily, all the
I/O that BDB does to its files gets cached in the Linux filesystem buffer
cache. This caching is redundant since BDB always does its own caching, and
it effectively makes the BDB environment consume twice as much memory as it
needs. Using O_DIRECT on I/Os disables the filesystem buffer cache for those
I/Os, thus freeing up a sizable chunk of memory.
The caching problem is particularly aggravated on Linux because the memory
manager doesn't give program pages higher priority than cache pages. So when
your system is tight on memory, the kernel will start swapping program data
pages before it starts reclaiming buffer cache pages, and application
performance plummets. (Possibly that indicates a kernel bug, or at least a
misfeature.)
Note that you must configure BerkeleyDB with --enable-o_direct to enable the
support, and you must add "set_flags DB_DIRECT_DB" to your DB_CONFIG to
enable it in a particular environment.
With this patch, a slapd that occupies 6.8GB on a system with 8GB of RAM can
run continuously without swapping, delivering a sustained 11,500
authentications per second. Without the patch, swapping starts when the
process hits the 4.5GB mark (because over 3GB of buffer cache is in use), and
performance drops to only *hundreds* of authentications per second.
--
-- Howard Chu
Chief Architect, Symas Corp.
http://www.symas.com
Director, Highland Sun
http://highlandsun.com/hyc
OpenLDAP Core Team
http://www.openldap.org/project/
--- dbinc/mp.h.orig 2006-09-07 14:31:58.000000000 -0700
+++ dbinc/mp.h 2007-01-06 19:14:56.000000000 -0800
@@ -378,6 +378,23 @@
#define BH_FREE_REUSE 0x02
#define BH_FREE_UNLOCKED 0x04
+#ifdef DIAG_MVCC
+#define BH_ALIGNED
+#define VM_PAGESIZE 4096
+#endif
+
+/* Linux O_DIRECT needs aligned buffers. 2.6 kernel allows 512 byte
+ * alignment, otherwise need page sized (4096).
+ */
+#if defined(linux) && !defined(BH_ALIGNED)
+#define BH_ALIGNED
+#ifdef LINUX_NEEDS_PAGE_ALIGNMENT
+#define VM_PAGESIZE 4096
+#else /* Linux 2.6+ */
+#define VM_PAGESIZE 512
+#endif
+#endif
+
/*
* BH --
* Buffer header.
@@ -404,7 +421,7 @@
roff_t td_off; /* MVCC: creating TXN_DETAIL offset. */
SH_CHAIN_ENTRY vc; /* MVCC: version chain. */
-#ifdef DIAG_MVCC
+#ifdef BH_ALIGNED
u_int16_t align_off; /* Alignment offset for diagnostics.*/
#endif
@@ -465,15 +482,14 @@
(dbc->txn != NULL && F_ISSET(dbc->txn, TXN_SNAPSHOT) && \
dbc->txn->td != NULL && __memp_skip_curadj(dbc, pgno))
-#if defined(DIAG_MVCC) && defined(HAVE_MPROTECT)
-#define VM_PAGESIZE 4096
-#define MVCC_BHSIZE(mfp, sz) do { \
+#ifdef BH_ALIGNED
+#define BHSIZE(mfp, sz) do { \
sz += VM_PAGESIZE + sizeof(BH); \
if (mfp->stat.st_pagesize < VM_PAGESIZE) \
sz += VM_PAGESIZE - mfp->stat.st_pagesize; \
} while (0)
-#define MVCC_BHALIGN(mfp, p) do { \
+#define BHALIGN(mfp, p) do { \
if (mfp != NULL) { \
BH *__bhp; \
void *__orig = (p); \
@@ -493,13 +509,19 @@
} \
} while (0)
-#define MVCC_BHUNALIGN(mfp, p) do { \
+#define BHUNALIGN(mfp, p) do { \
if ((mfp) != NULL) { \
BH *bhp = (BH *)(p); \
(p) = ((u_int8_t *)bhp - bhp->align_off); \
} \
} while (0)
+#else
+#define BHSIZE(mfp, sz) do {} while (0)
+#define BHALIGN(mfp, p) do {} while (0)
+#define BHUNALIGN(mfp, p) do {} while (0)
+#endif
+#if defined(DIAG_MVCC) && defined(HAVE_MPROTECT)
#ifdef linux
#define MVCC_MPROTECT(buf, sz, mode) do { \
int __ret = mprotect((buf), (sz), (mode)); \
@@ -513,11 +535,7 @@
} \
} while (0)
#endif /* linux */
-
-#else /* defined(DIAG_MVCC) && defined(HAVE_MPROTECT) */
-#define MVCC_BHSIZE(mfp, sz) do {} while (0)
-#define MVCC_BHALIGN(mfp, p) do {} while (0)
-#define MVCC_BHUNALIGN(mfp, p) do {} while (0)
+#else
#define MVCC_MPROTECT(buf, size, mode) do {} while (0)
#endif
--- mp/mp_alloc.c.orig 2006-09-07 14:32:03.000000000 -0700
+++ mp/mp_alloc.c 2007-01-06 19:14:56.000000000 -0800
@@ -66,7 +66,7 @@
if (mfp != NULL) {
len = SSZA(BH, buf) + mfp->stat.st_pagesize;
/* Add space for alignment padding for MVCC diagnostics. */
- MVCC_BHSIZE(mfp, len);
+ BHSIZE(mfp, len);
}
MPOOL_REGION_LOCK(dbenv, infop);
@@ -91,10 +91,10 @@
c_mp->stat.st_pages++;
MPOOL_REGION_UNLOCK(dbenv, infop);
/*
- * For MVCC diagnostics, align the pointer so that the buffer
+ * If necessary, align the pointer so that the buffer
* starts on a page boundary.
*/
- MVCC_BHALIGN(mfp, p);
+ BHALIGN(mfp, p);
found: if (offsetp != NULL)
*offsetp = R_OFFSET(infop, p);
@@ -447,7 +447,7 @@
MPOOLFILE *mfp;
void *buf;
{
- MVCC_BHUNALIGN(mfp, buf);
+ BHUNALIGN(mfp, buf);
COMPQUIET(mfp, NULL);
__db_shalloc_free(infop, buf);
}
--- mp/mp_fget.c.orig 2006-09-13 09:22:42.000000000 -0700
+++ mp/mp_fget.c 2007-01-06 19:14:56.000000000 -0800
@@ -708,7 +708,7 @@
* the hash bucket's priority.
*/
/*lint --e{668} (flexelint: bhp cannot be NULL). */
-#ifdef DIAG_MVCC
+#ifdef BH_ALIGNED
memset(bhp, 0, SSZ(BH, align_off));
#else
memset(bhp, 0, sizeof(BH));
