Re: Self-ballooning question / cache issue

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, May 02, 2012 at 10:51:12AM -0700, Dan Magenheimer wrote:
> > From: Jana Saout [mailto:jana@saout.de]
> > 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@oracle.com>

Looks good. Can you resend it with the right tags to
xen-devel and lkml and to me please?
> 
> > > 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@saout.de>
> > 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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