Re: [RFC/T/D][PATCH 2/2] Linux/Guest cooperative unmapped page cachecontrol

From: Balbir Singh
Date: Tue Jun 15 2010 - 06:19:09 EST

* Avi Kivity <avi@xxxxxxxxxx> [2010-06-15 12:44:31]:

> On 06/15/2010 10:49 AM, Balbir Singh wrote:
> >
> >>All we need is to select the right page to drop.
> >>
> >Do we need to drop to the granularity of the page to drop? I think
> >figuring out the class of pages and making sure that we don't write
> >our own reclaim logic, but work with what we have to identify the
> >class of pages is a good start.
> Well, the class of pages are 'pages that are duplicated on the
> host'. Unmapped page cache pages are 'pages that might be
> duplicated on the host'. IMO, that's not close enough.

Agreed, but what happens in reality with the code is that it drops
not-so-frequently-used cache (still reusing the reclaim mechanism),
but prioritizing cached memory.

> >>How can the host tell if there is duplication? It may know it has
> >>some pagecache, but it has no idea whether or to what extent guest
> >>pagecache duplicates host pagecache.
> >>
> >Well it is possible in host user space, I for example use memory
> >cgroup and through the stats I have a good idea of how much is duplicated.
> >I am ofcourse making an assumption with my setup of the cached mode,
> >that the data in the guest page cache and page cache in the cgroup
> >will be duplicated to a large extent. I did some trivial experiments
> >like drop the data from the guest and look at the cost of bringing it
> >in and dropping the data from both guest and host and look at the
> >cost. I could see a difference.
> >
> >Unfortunately, I did not save the data, so I'll need to redo the
> >experiment.
> I'm sure we can detect it experimentally, but how do we do it
> programatically at run time (without dropping all the pages).
> Situations change, and I don't think we can infer from a few
> experiments that we'll have a similar amount of sharing. The cost
> of an incorrect decision is too high IMO (not that I think the
> kernel always chooses the right pages now, but I'd like to avoid
> regressions from the unvirtualized state).
> btw, when running with a disk controller that has a very large
> cache, we might also see duplication between "guest" and host. So,
> if this is a good idea, it shouldn't be enabled just for
> virtualization, but for any situation where we have a sizeable cache
> behind us.

It depends, once the disk controller has the cache and the pages in
the guest are not-so-frequently-used we can drop them. Please remember
we still use the LRU to identify these pages.

> >>It doesn't, really. The host only has aggregate information about
> >>itself, and no information about the guest.
> >>
> >>Dropping duplicate pages would be good if we could identify them.
> >>Even then, it's better to drop the page from the host, not the
> >>guest, unless we know the same page is cached by multiple guests.
> >>
> >On the exact pages to drop, please see my comments above on the class
> >of pages to drop.
> Well, we disagree about that. There is some value in dropping
> duplicated pages (not always), but that's not what the patch does.
> It drops unmapped pagecache pages, which may or may not be
> duplicated.
> >There are reasons for wanting to get the host to cache the data
> There are also reasons to get the guest to cache the data - it's
> more efficient to access it in the guest.
> >Unless the guest is using cache = none, the data will still hit the
> >host page cache
> >The host can do a better job of optimizing the writeouts
> True, especially for non-raw storage. But even there we have to
> fsync all the time to keep the metadata right.
> >>But why would the guest voluntarily drop the cache? If there is no
> >>memory pressure, dropping caches increases cpu overhead and latency
> >>even if the data is still cached on the host.
> >>
> >So, there are basically two approaches
> >
> >1. First patch, proactive - enabled by a boot option
> >2. When ballooned, we try to (please NOTE try to) reclaim cached pages
> >first. Failing which, we go after regular pages in the alloc_page()
> >call in the balloon driver.
> Doesn't that mean you may evict a RU mapped page ahead of an LRU
> unmapped page, just in the hope that it is double-cached?
> Maybe we need the guest and host to talk to each other about which
> pages to keep.

Yeah.. I guess that falls into the domain of CMM.

> >>>2. Drop the cache on either a special balloon option, again the host
> >>>knows it caches that very same information, so it prefers to free that
> >>>up first.
> >>Dropping in response to pressure is good. I'm just not convinced
> >>the patch helps in selecting the correct page to drop.
> >>
> >That is why I've presented data on the experiments I've run and
> >provided more arguments to backup the approach.
> I'm still unconvinced, sorry.

The reason for making this optional is to let the administrators
decide how they want to use the memory in the system. In some
situations it might be a big no-no to waste memory, in some cases it
might be acceptable.

Three Cheers,
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