Re: [Xen-devel] Self-ballooning question / cache issue

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Jana Saout [mailto:jana@xxxxxxxx]
> Subject: Re: [Xen-devel] Self-ballooning question / cache issue
> 

Hi Jana --

Since you have tested this patch and have found it useful, and
since its use is entirely optional, it is OK with me for it to
be upstreamed at the next window.  Konrad cc'ed.

You will need to add a Signed-off-by line to the patch
but other than that you can consider it

Acked-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>

> > Your idea of the tunable is interesting (and patches are always
> > welcome!) but I am skeptical that it will solve the problem
> > since I would guess the Linux kernel is shrinking dcache
> > proportional to the size of the page cache.  So adding more
> > RAM with your "user-specified amount of pages that is
> > added on top of the computed target number of pages",
> > the RAM will still be shared across all caches and only
> > some small portion of the added RAM will likely be used
> > for dcache.
> 
> That's true.  In fact, I have to add about 1 GB of memory in order to
> keep the relevant dcache / inode cache entries to stay in the cache.
> When I do that the largest portion of memory is still eaten up by the
> regular page cache.  So this is more of a workaround than a solution,
> but for now it works.
> 
> I've attached the simple patch I've whipped up below.
> 
> > However, if you have a chance to try it, I would be interested
> > in your findings.  Note that you already can set a
> > permanent floor for selfballooning ("min_usable_mb") or,
> > of course, just turn off selfballooning altogether.
> 
> Sure, that's always a possibility.  However, the VM already had an
> overly large amount of memory before to avoid the problem.  Now it runs
> with less memory (still a bit more than required), and when a load spike
> comes, it can quickly balloon up, which is exactly what I was looking
> for.
> 
>       Jana
> 
> ----
> Author: Jana Saout <jana@xxxxxxxx>
> Date:   Sun Apr 29 22:09:29 2012 +0200
> 
>     Add selfballoning memory reservation tunable.
> 
> diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
> index 146c948..7d041cb 100644
> --- a/drivers/xen/xen-selfballoon.c
> +++ b/drivers/xen/xen-selfballoon.c
> @@ -105,6 +105,12 @@ static unsigned int selfballoon_interval __read_mostly = 
> 5;
>   */
>  static unsigned int selfballoon_min_usable_mb;
> 
> +/*
> + * Amount of RAM in MB to add to the target number of pages.
> + * Can be used to reserve some more room for caches and the like.
> + */
> +static unsigned int selfballoon_reserved_mb;
> +
>  static void selfballoon_process(struct work_struct *work);
>  static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
> 
> @@ -217,7 +223,8 @@ static void selfballoon_process(struct work_struct *work)
>               cur_pages = totalram_pages;
>               tgt_pages = cur_pages; /* default is no change */
>               goal_pages = percpu_counter_read_positive(&vm_committed_as) +
> -                             totalreserve_pages;
> +                             totalreserve_pages +
> +                             MB2PAGES(selfballoon_reserved_mb);
>  #ifdef CONFIG_FRONTSWAP
>               /* allow space for frontswap pages to be repatriated */
>               if (frontswap_selfshrinking && frontswap_enabled)
> @@ -397,6 +404,30 @@ static DEVICE_ATTR(selfballoon_min_usable_mb, S_IRUGO | 
> S_IWUSR,
>                  show_selfballoon_min_usable_mb,
>                  store_selfballoon_min_usable_mb);
> 
> +SELFBALLOON_SHOW(selfballoon_reserved_mb, "%d\n",
> +                             selfballoon_reserved_mb);
> +
> +static ssize_t store_selfballoon_reserved_mb(struct device *dev,
> +                                          struct device_attribute *attr,
> +                                          const char *buf,
> +                                          size_t count)
> +{
> +     unsigned long val;
> +     int err;
> +
> +     if (!capable(CAP_SYS_ADMIN))
> +             return -EPERM;
> +     err = strict_strtoul(buf, 10, &val);
> +     if (err)
> +             return -EINVAL;
> +     selfballoon_reserved_mb = val;
> +     return count;
> +}
> +
> +static DEVICE_ATTR(selfballoon_reserved_mb, S_IRUGO | S_IWUSR,
> +                show_selfballoon_reserved_mb,
> +                store_selfballoon_reserved_mb);
> +
> 
>  #ifdef CONFIG_FRONTSWAP
>  SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
> @@ -480,6 +511,7 @@ static struct attribute *selfballoon_attrs[] = {
>       &dev_attr_selfballoon_downhysteresis.attr,
>       &dev_attr_selfballoon_uphysteresis.attr,
>       &dev_attr_selfballoon_min_usable_mb.attr,
> +     &dev_attr_selfballoon_reserved_mb.attr,
>  #ifdef CONFIG_FRONTSWAP
>       &dev_attr_frontswap_selfshrinking.attr,
>       &dev_attr_frontswap_hysteresis.attr,
> 
> 

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