Re: How to controll Buffers to be dilligently reclaimed?

[Michal Hocko <mhocko@suse.cz>]

On Wed 18-02-15 19:44:06, Cheng Rk wrote:
> 
> 
> On Wednesday, February 18, 2015 6:23 AM, Michal Hocko <mhocko@suse.cz> wrote:
> On Fri 13-02-15 09:52:16, Cheng Rk wrote:
> 
> > As per Documentation/sysctl/vm.txt the knob doesn't affect the page
> > cache reclaim but rather inode vs. dentry reclaim.
> 
> 
> So do you think is it worth to work on something to give pressure similar
> to vm.vfs_cache_pressure to vfs inode & dentry cache?
> 
> I am looking for an effect to let the kernel more aggressively reclaim
> memory from Buffers,

more aggressively than what? Anonymous memory, other types of caches?
To be honest I do not see why we should treat buffers any different from
any other cache. So far it is not clear what might be the issue you are
seeing but I would suspect that too many buffers is not the primary one.
It is hard to say anything more without any specific numbers, though.
 
> By reading fs/super.c:prune_super I've also realized taht, which is the
> only place referening sysctl_vfs_cache_pressure,
> that block_devices' inode are in "bdev" mount, its super_block just
> have nr_cached_objects as NULL,
> s_nr_dentry_unused and s_nr_inodes_unused both 0, get total_objects to be
> reclaimed is 0;
> 
> So is why sysctl_vfs_cache_pressure doesn't give pressure to Buffers,
> 
> 
>          if (sb->s_op && sb->s_op->nr_cached_objects)
>                    fs_objects = sb->s_op->nr_cached_objects(sb);
> 
>   total_objects = sb->s_nr_dentry_unused +
>                                          sb->s_nr_inodes_unused + fs_objects;
> 
>   total_objects = (total_objects / 100) * sysctl_vfs_cache_pressure;
>   drop_super(sb);
> 
> 
> In crash, I got to know this block_device (253:2, or /dev/dm-2)has
> 10536805 pages mapped, that is the 40GB memory in Buffers,

I still do not see why is that a problem. They should get reclaimed on
demand.

> I wonder is there a sysctl can controll this to be reclaimed earlier?

I do not know about any.

[...]
-- 
Michal Hocko
SUSE Labs

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