Re: [PATCH v2] Make vm_max_readahead configurable at run-time

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Feb 10, 2010 at 09:52:40PM +0800, Nikanth Karthikesan wrote:
> On Wednesday 10 February 2010 16:35:51 Wu Fengguang wrote:
> > Nikanth,
> > 
> > > Make vm_max_readahead configurable at run-time. Expose a sysctl knob
> > > in procfs to change it. This would ensure that new disks added would
> > > use this value as their default read_ahead_kb.
> > 
> > Do you have use case, or customer demand for it?
> > 
> 
> No body requested for it. But when doing some performance testing with 
> readahead_kb re-compiling would be a pain, and thought that having a 
> configurable default might be useful.

I wonder why you need to recompile kernel in the tests.
There are three interfaces to change readahead size in runtime:

        blockdev --setra 1024 /dev/sda
        echo 512 > /sys/block/*/queue/read_ahead_kb
        echo 512 > /sys/devices/virtual/bdi/*/read_ahead_kb

> > > Also filesystems which use default_backing_dev_info would also
> > > use this new value, even if they were already mounted.
> > >
> > > Currently xfs, btrfs, nilfs, raw, mtd use the default_backing_dev_info.
> > 
> > This sounds like bad interface, in that users will be confused by the
> > tricky details of "works for new devices" and "works for some fs".
> > 
> > One more tricky point is, btrfs/md/dm readahead size may not be
> > influenced if some of the component disks are hot added.
> > 
> > So this patch is only going to work for hot-plugged disks that
> > contains _standalone_ filesystem. Is this typical use case in servers?
> > 
> 
> Yes, it would work only if the top-level disk is hot-plugged/created.
 
Or maybe what you really want is a kernel parameter for setting the
default readahead size at boot time?

In another thread, Christian Ehrhardt recommended to add a config
option for it. If you like it, I can also do the kernel parameter
by the way.

Thanks,
Fengguang

> > > Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
> > >
> > > ---
> > >
> > > Index: linux-2.6/block/blk-core.c
> > > ===================================================================
> > > --- linux-2.6.orig/block/blk-core.c
> > > +++ linux-2.6/block/blk-core.c
> > > @@ -499,7 +499,7 @@ struct request_queue *blk_alloc_queue_no
> > >  	q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
> > >  	q->backing_dev_info.unplug_io_data = q;
> > >  	q->backing_dev_info.ra_pages =
> > > -			(VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
> > > +			(vm_max_readahead_kb * 1024) / PAGE_CACHE_SIZE;
> > >  	q->backing_dev_info.state = 0;
> > >  	q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
> > >  	q->backing_dev_info.name = "block";
> > > Index: linux-2.6/fs/fuse/inode.c
> > > ===================================================================
> > > --- linux-2.6.orig/fs/fuse/inode.c
> > > +++ linux-2.6/fs/fuse/inode.c
> > > @@ -870,7 +870,7 @@ static int fuse_bdi_init(struct fuse_con
> > >  	int err;
> > >
> > >  	fc->bdi.name = "fuse";
> > > -	fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
> > > +	fc->bdi.ra_pages = (vm_max_readahead_kb * 1024) / PAGE_CACHE_SIZE;
> > >  	fc->bdi.unplug_io_fn = default_unplug_io_fn;
> > >  	/* fuse does it's own writeback accounting */
> > >  	fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB;
> > > Index: linux-2.6/include/linux/mm.h
> > > ===================================================================
> > > --- linux-2.6.orig/include/linux/mm.h
> > > +++ linux-2.6/include/linux/mm.h
> > > @@ -1188,7 +1188,11 @@ int write_one_page(struct page *page, in
> > >  void task_dirty_inc(struct task_struct *tsk);
> > >
> > >  /* readahead.c */
> > > -#define VM_MAX_READAHEAD	128	/* kbytes */
> > > +#define INITIAL_VM_MAX_READAHEAD_KB	128
> > > +extern unsigned long vm_max_readahead_kb;
> > > +
> > > +int sysctl_vm_max_readahead_kb_handler(struct ctl_table *table, int
> > > write, +		void __user *buffer, size_t *length, loff_t *ppos);
> > >
> > >  int force_page_cache_readahead(struct address_space *mapping, struct
> > > file *filp, pgoff_t offset, unsigned long nr_to_read);
> > > Index: linux-2.6/mm/backing-dev.c
> > > ===================================================================
> > > --- linux-2.6.orig/mm/backing-dev.c
> > > +++ linux-2.6/mm/backing-dev.c
> > > @@ -18,7 +18,8 @@ EXPORT_SYMBOL(default_unplug_io_fn);
> > >
> > >  struct backing_dev_info default_backing_dev_info = {
> > >  	.name		= "default",
> > > -	.ra_pages	= VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
> > > +	.ra_pages	= INITIAL_VM_MAX_READAHEAD_KB
> > > +					>> (PAGE_CACHE_SHIFT - 10),
> > >  	.state		= 0,
> > >  	.capabilities	= BDI_CAP_MAP_COPY,
> > >  	.unplug_io_fn	= default_unplug_io_fn,
> > > Index: linux-2.6/mm/readahead.c
> > > ===================================================================
> > > --- linux-2.6.orig/mm/readahead.c
> > > +++ linux-2.6/mm/readahead.c
> > > @@ -17,6 +17,19 @@
> > >  #include <linux/pagevec.h>
> > >  #include <linux/pagemap.h>
> > >
> > > +unsigned long vm_max_readahead_kb = INITIAL_VM_MAX_READAHEAD_KB;
> > > +
> > > +int sysctl_vm_max_readahead_kb_handler(struct ctl_table *table, int
> > > write, +		void __user *buffer, size_t *length, loff_t *ppos)
> > > +{
> > > +	proc_doulongvec_minmax(table, write, buffer, length, ppos);
> > > +
> > > +	default_backing_dev_info.ra_pages =
> > > +			vm_max_readahead_kb >> (PAGE_CACHE_SHIFT - 10);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > >  /*
> > >   * Initialise a struct file's readahead state.  Assumes that the caller
> > > has * memset *ra to zero.
> > > Index: linux-2.6/kernel/sysctl.c
> > > ===================================================================
> > > --- linux-2.6.orig/kernel/sysctl.c
> > > +++ linux-2.6/kernel/sysctl.c
> > > @@ -1273,7 +1273,13 @@ static struct ctl_table vm_table[] = {
> > >  		.extra2		= &one,
> > >  	},
> > >  #endif
> > > -
> > > +	{
> > > +		.procname	= "max_readahead_kb",
> > > +		.data		= &vm_max_readahead_kb,
> > > +		.maxlen		= sizeof(vm_max_readahead_kb),
> > > +		.mode		= 0644,
> > > +		.proc_handler	= sysctl_vm_max_readahead_kb_handler,
> > > +	},
> > >  	{ }
> > >  };
> > >
> > > Index: linux-2.6/Documentation/sysctl/vm.txt
> > > ===================================================================
> > > --- linux-2.6.orig/Documentation/sysctl/vm.txt
> > > +++ linux-2.6/Documentation/sysctl/vm.txt
> > > @@ -31,6 +31,7 @@ Currently, these files are in /proc/sys/
> > >  - laptop_mode
> > >  - legacy_va_layout
> > >  - lowmem_reserve_ratio
> > > +- max_readahead_kb
> > >  - max_map_count
> > >  - memory_failure_early_kill
> > >  - memory_failure_recovery
> > > @@ -263,6 +264,18 @@ The minimum value is 1 (1/1 -> 100%).
> > >
> > >  ==============================================================
> > >
> > > +max_readahead_kb:
> > > +
> > > +This file contains the default maximum readahead that would be
> > > +used, when new disks would be added to the system.
> > > +
> > > +Also filesystems which use default_backing_dev_info would also
> > > +use this new value, even if they were already mounted.
> > > +
> > > +xfs, btrfs, nilfs, raw, mtd use the default_backing_dev_info.
> > > +
> > > +==============================================================
> > > +
> > >  max_map_count:
> > >
> > >  This file contains the maximum number of memory map areas a process
> >