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

[Ying Han <yinghan@google.com>]

On Mon, May 2, 2011 at 6:24 PM, 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 can be used to make the VM free up memory, for when an extra
> workload is to be added to a system, or to temporarily reduce the
> memory use of a virtual machine. In this application, extra_free_kbytes
> would be raised temporarily and reduced again later.  The workload
> management system could also monitor the current workloads with
> reduced memory, to verify that there really is memory space for
> an additional workload, before starting it.
>
> It may also be 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.
>
> I realize nobody really likes this solution to their particular
> issue, but it is hard to deny the simplicity - especially
> considering that this one knob could solve three different issues
> and is fairly simple to understand.

Hi Rik:

In general, i would wonder what's the specific use case requiring the
extra tunable in the kernel. I think I can see the point you are
making,  but it would be hard for admin to adjust the per-zone
extra_free_bytes based on the workload.

In memcg case, we are proposing to add the "high_wmark_distance"
per-memcg, which allows us to tune the high/low_wmark per-memcg
background reclaim. One of the usecase shares w/ your comment which we
want to proactively reclaim memory for launching new jobs. More on
that, this tunable gives us more targeting reclaim. While looking at
this patch, some motivation are common but not the same. At least in
memcg, it might be hard to use per-zone extra_free_bytes to serve the
same purpose because the tunable is for every zone of the system. So
we might start soft_limit reclaim on all the memcgs on all the zones.
Which sounds overkill if we can pick few memcgs to reclaim from.

In non-memcg case, we can imagine lifting the low_wmark might
introduce more background reclaim, which in turn less direct reclaim.
But tuning the knob would be tricky and we need data to support that.

Thanks

--Ying

>
> Signed-off-by: Rik van Riel<riel@redhat.com>
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index c0bb324..feecc1a 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -95,6 +95,7 @@ extern char core_pattern[];
>  extern unsigned int core_pipe_limit;
>  extern int pid_max;
>  extern int min_free_kbytes;
> +extern int extra_free_kbytes;
>  extern int pid_max_min, pid_max_max;
>  extern int sysctl_drop_caches;
>  extern int percpu_pagelist_fraction;
> @@ -1173,6 +1174,14 @@ 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,
> +               .extra1         = &zero,
> +       },
> +       {
>                .procname       = "percpu_pagelist_fraction",
>                .data           = &percpu_pagelist_fraction,
>                .maxlen         = sizeof(percpu_pagelist_fraction),
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9f8a97b..b85dcb1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -172,8 +172,20 @@ static char * const zone_names[MAX_NR_ZONES] = {
>         "Movable",
>  };
>
> +/*
> + * Try to keep at least this much lowmem free.  Do not allow normal
> + * allocations below this point, only high priority ones. Automatically
> + * tuned according to the amount of memory in the system.
> + */
>  int min_free_kbytes = 1024;
>
> +/*
> + * Extra memory for the system to try freeing. Used to temporarily
> + * free memory, to make space for new workloads. Anyone can allocate
> + * down to the min watermarks controlled by min_free_kbytes above.
> + */
> +int extra_free_kbytes = 0;
> +
>  static unsigned long __meminitdata nr_kernel_pages;
>  static unsigned long __meminitdata nr_all_pages;
>  static unsigned long __meminitdata dma_reserve;
> @@ -4999,6 +5011,7 @@ static void setup_per_zone_lowmem_reserve(void)
>  void setup_per_zone_wmarks(void)
>  {
>        unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);
> +       unsigned long pages_low = extra_free_kbytes >> (PAGE_SHIFT - 10);
>        unsigned long lowmem_pages = 0;
>        struct zone *zone;
>        unsigned long flags;
> @@ -5010,11 +5023,14 @@ void setup_per_zone_wmarks(void)
>        }
>
>        for_each_zone(zone) {
> -               u64 tmp;
> +               u64 min, low;
>
>                spin_lock_irqsave(&zone->lock, flags);
> -               tmp = (u64)pages_min * zone->present_pages;
> -               do_div(tmp, lowmem_pages);
> +               min = (u64)pages_min * zone->present_pages;
> +               do_div(min, lowmem_pages);
> +               low = (u64)pages_low * zone->present_pages;
> +               do_div(low, vm_total_pages);
> +
>                if (is_highmem(zone)) {
>                        /*
>                         * __GFP_HIGH and PF_MEMALLOC allocations usually don't
> @@ -5038,11 +5054,13 @@ void setup_per_zone_wmarks(void)
>                         * If it's a lowmem zone, reserve a number of pages
>                         * proportionate to the zone's size.
>                         */
> -                       zone->watermark[WMARK_MIN] = tmp;
> +                       zone->watermark[WMARK_MIN] = min;
>                }
>
> -               zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + (tmp >> 2);
> -               zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
> +               zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) +
> +                                       low + (min >> 2);
> +               zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) +
> +                                       low + (min >> 1);
>                setup_zone_migrate_reserve(zone);
>                spin_unlock_irqrestore(&zone->lock, flags);
>        }
> @@ -5139,7 +5157,7 @@ module_init(init_per_zone_wmark_min)
>  /*
>  * min_free_kbytes_sysctl_handler - just a wrapper around proc_dointvec() so
>  *     that we can call two helper functions whenever min_free_kbytes
> - *     changes.
> + *     or extra_free_kbytes changes.
>  */
>  int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
>        void __user *buffer, size_t *length, loff_t *ppos)
>
>

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