Re: [PATCH 0/4] linux-2.6-block: deactivating pagecache for benchmarks

[Andrew Morton <akpm@osdl.org>]

Dirk Henning Gerdes <mail@dirk-gerdes.de> wrote:
>
>  For doing benchmarks on the I/O-Schedulers, I thought it would be very
>  useful to disable the pagecache.

That's an FAQ.   Something like this?


From: Andrew Morton <akpm@osdl.org>

Add /proc/sys/vm/drop-pagecache.  When written to, this will cause the kernel
to discard as much pagecache and reclaimable slab objects as it can.

It won't drop dirty data, so the user should run `sync' first.

Caveats:

a) Holds inode_lock for exorbitant amounts of time.

b) Needs to be taught about NUMA nodes: propagate these all the way through
   so the discarding can be controlled on a per-node basis.

c) The pagecache shrinking and slab shrinking should probably have separate
   controls.


Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 fs/Makefile            |    2 -
 fs/drop-pagecache.c    |   62 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h     |    5 +++
 include/linux/sysctl.h |    1 
 kernel/sysctl.c        |    9 +++++++
 mm/truncate.c          |    1 
 mm/vmscan.c            |    3 --
 7 files changed, 79 insertions(+), 4 deletions(-)

diff -puN /dev/null fs/drop-pagecache.c
--- /dev/null	2003-09-15 06:40:47.000000000 -0700
+++ devel-akpm/fs/drop-pagecache.c	2005-12-01 17:20:55.000000000 -0800
@@ -0,0 +1,62 @@
+/*
+ * Implement the manual drop-all-pagecache function
+ */
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/writeback.h>
+#include <linux/sysctl.h>
+#include <linux/gfp.h>
+
+static void drop_pagecache_sb(struct super_block *sb)
+{
+	struct inode *inode;
+
+	spin_lock(&inode_lock);
+	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+		if (inode->i_state & (I_FREEING|I_WILL_FREE))
+			continue;
+		invalidate_inode_pages(inode->i_mapping);
+	}
+	spin_unlock(&inode_lock);
+}
+
+static void drop_pagecache(void)
+{
+	struct super_block *sb;
+
+	spin_lock(&sb_lock);
+restart:
+	list_for_each_entry(sb, &super_blocks, s_list) {
+		sb->s_count++;
+		spin_unlock(&sb_lock);
+		down_read(&sb->s_umount);
+		if (sb->s_root)
+			drop_pagecache_sb(sb);
+		up_read(&sb->s_umount);
+		spin_lock(&sb_lock);
+		if (__put_super_and_need_restart(sb))
+			goto restart;
+	}
+	spin_unlock(&sb_lock);
+	printk("shrunk pagecache\n");
+}
+
+static void drop_slab(void)
+{
+	int nr_objects;
+
+	do {
+		nr_objects = shrink_slab(1000, GFP_KERNEL, 1000);
+		printk("shrunk %d cache objects\n", nr_objects);
+	} while (nr_objects > 10);
+}
+
+int drop_pagecache_sysctl_handler(ctl_table *table, int write,
+	struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
+{
+	drop_pagecache();
+	drop_slab();
+	return 0;
+}
diff -puN fs/Makefile~drop-pagecache fs/Makefile
--- devel/fs/Makefile~drop-pagecache	2005-12-01 16:41:22.000000000 -0800
+++ devel-akpm/fs/Makefile	2005-12-01 16:41:22.000000000 -0800
@@ -10,7 +10,7 @@ obj-y :=	open.o read_write.o file_table.
 		ioctl.o readdir.o select.o fifo.o locks.o dcache.o inode.o \
 		attr.o bad_inode.o file.o filesystems.o namespace.o aio.o \
 		seq_file.o xattr.o libfs.o fs-writeback.o mpage.o direct-io.o \
-		ioprio.o pnode.o
+		ioprio.o pnode.o drop-pagecache.o
 
 obj-$(CONFIG_INOTIFY)		+= inotify.o
 obj-$(CONFIG_EPOLL)		+= eventpoll.o
diff -puN include/linux/mm.h~drop-pagecache include/linux/mm.h
--- devel/include/linux/mm.h~drop-pagecache	2005-12-01 16:41:22.000000000 -0800
+++ devel-akpm/include/linux/mm.h	2005-12-01 17:01:57.000000000 -0800
@@ -1078,5 +1078,10 @@ int in_gate_area_no_task(unsigned long a
 /* /proc/<pid>/oom_adj set to -17 protects from the oom-killer */
 #define OOM_DISABLE -17
 
+int drop_pagecache_sysctl_handler(struct ctl_table *, int, struct file *,
+					void __user *, size_t *, loff_t *);
+int shrink_slab(unsigned long scanned, gfp_t gfp_mask,
+			unsigned long lru_pages);
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff -puN include/linux/sysctl.h~drop-pagecache include/linux/sysctl.h
--- devel/include/linux/sysctl.h~drop-pagecache	2005-12-01 16:41:22.000000000 -0800
+++ devel-akpm/include/linux/sysctl.h	2005-12-01 16:41:22.000000000 -0800
@@ -182,6 +182,7 @@ enum
 	VM_LEGACY_VA_LAYOUT=27, /* legacy/compatibility virtual address space layout */
 	VM_SWAP_TOKEN_TIMEOUT=28, /* default time for token time out */
 	VM_SWAP_PREFETCH=29,	/* int: amount to swap prefetch */
+	VM_DROP_PAGECACHE=30,	/* int: nuke lots of pagecache */
 };
 
 
diff -puN kernel/sysctl.c~drop-pagecache kernel/sysctl.c
--- devel/kernel/sysctl.c~drop-pagecache	2005-12-01 16:41:22.000000000 -0800
+++ devel-akpm/kernel/sysctl.c	2005-12-01 16:41:22.000000000 -0800
@@ -783,6 +783,15 @@ static ctl_table vm_table[] = {
 		.strategy	= &sysctl_intvec,
 	},
 	{
+		.ctl_name	= VM_DROP_PAGECACHE,
+		.procname	= "drop-pagecache",
+		.data		= NULL,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= drop_pagecache_sysctl_handler,
+		.strategy	= &sysctl_intvec,
+	},
+	{
 		.ctl_name	= VM_MIN_FREE_KBYTES,
 		.procname	= "min_free_kbytes",
 		.data		= &min_free_kbytes,
diff -puN mm/truncate.c~drop-pagecache mm/truncate.c
--- devel/mm/truncate.c~drop-pagecache	2005-12-01 16:49:06.000000000 -0800
+++ devel-akpm/mm/truncate.c	2005-12-01 16:49:13.000000000 -0800
@@ -256,7 +256,6 @@ unlock:
 				break;
 		}
 		pagevec_release(&pvec);
-		cond_resched();
 	}
 	return ret;
 }
diff -puN mm/vmscan.c~drop-pagecache mm/vmscan.c
--- devel/mm/vmscan.c~drop-pagecache	2005-12-01 16:58:30.000000000 -0800
+++ devel-akpm/mm/vmscan.c	2005-12-01 17:00:39.000000000 -0800
@@ -181,8 +181,7 @@ EXPORT_SYMBOL(remove_shrinker);
  *
  * Returns the number of slab objects which we shrunk.
  */
-static int shrink_slab(unsigned long scanned, gfp_t gfp_mask,
-			unsigned long lru_pages)
+int shrink_slab(unsigned long scanned, gfp_t gfp_mask, unsigned long lru_pages)
 {
 	struct shrinker *shrinker;
 	int ret = 0;
_