Re: [PATCH -v2 -mm] add extra free kbytes tunable

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 1 Sep 2011 15:26:50 -0400
Rik van Riel <riel@redhat.com> wrote:

> Add a userspace visible knob to tell the VM to keep an extra amount
> of memory free, by increasing the gap between each zone's min and
> low watermarks.
> 
> This is useful for realtime applications that call system
> calls and have a bound on the number of allocations that happen
> in any short time period.  In this application, extra_free_kbytes
> would be left at an amount equal to or larger than than the
> maximum number of allocations that happen in any burst.
> 
> It may also be useful to reduce the memory use of virtual
> machines (temporarily?), in a way that does not cause memory
> fragmentation like ballooning does.
> 
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -96,6 +96,7 @@ extern char core_pattern[];
>  extern unsigned int core_pipe_limit;
>  extern int pid_max;
>  extern int min_free_kbytes;
> +extern int extra_free_kbytes;

No externs in C, please.  Feel free to fix min_free_kbytes while you're
there ;)

swap.h is a common place to declare these things.  mmzone.h would make
sense too.


>  extern int pid_max_min, pid_max_max;
>  extern int sysctl_drop_caches;
>  extern int percpu_pagelist_fraction;
> @@ -1189,6 +1190,14 @@ static struct ctl_table vm_table[] = {
>  		.extra1		= &zero,
>  	},
>  	{
> +		.procname	= "extra_free_kbytes",
> +		.data		= &extra_free_kbytes,
> +		.maxlen		= sizeof(extra_free_kbytes),
> +		.mode		= 0644,
> +		.proc_handler	= min_free_kbytes_sysctl_handler,

Lazy.  The function should be renamed to accurately reflect its role.

> +		.extra1		= &zero,
> +	},
> +	{
>  		.procname	= "percpu_pagelist_fraction",
>  		.data		= &percpu_pagelist_fraction,
>  		.maxlen		= sizeof(percpu_pagelist_fraction),
>
> ...
>
> +int extra_free_kbytes = 0;

I'm inclined to agree with checkpatch here - it's just noise.