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

Rob Landley
2005-12-05T16:02:12Z

On Thursday 01 December 2005 19:25, Andrew Morton wrote:
> 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.

This is deeply, deeply cool.

> Caveats:
>
> a) Holds inode_lock for exorbitant amounts of time.

Voluntary preemption point, maybe?

> 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.

It could care about _what_ you write to it, maybe? (The first byte,
anyway...)

>
> 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,

So what _does_ it do when you read from it?

> + .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();

Why drop that line? (I don't follow...)

Rob
--
Steve Ballmer: Innovation! Inigo Montoya: You keep using that word.
I do not think it means what you think it means.