RE: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Hi Wei --

I'm not worried about the overhead of the splintering, I'm
worried about the "hidden overhead" everytime a "silent
splinter" is used.

Let's assume three scenarios (and for now use 2MB pages though
the same concerns can be extended to 1GB and/or mixed 2MB/1GB):

A) DB code assumes 2MB pages, OS assumes 2MB pages, Xen provides
   only 2MB pages (no splintering occurs)
B) DB code assumes 2MB pages, OS assumes 2MB pages, Xen provides
   only 4KB pages (because of fragmentation, all 2MB pages have
   been splintered)
C) DB code assumes 4KB pages, OS assumes 4KB pages, Xen provides
   4KB pages

Now run some benchmarks.  Clearly one would assume that A is
faster than both B and C.  The question is: Is B faster or slower
than C?

If B is always faster than C, then I have less objection to
"silent splintering".  But if B is sometimes (or maybe always?)
slower than C, that's a big issue because a user has gone through
the effort of choosing a better-performing system configuration
for their software (2MB DB on 2MB OS), but it actually performs
worse than if they had chosen the "lower performing" configuration.
And, worse, it will likely degrade across time so performance
might be fine when the 2MB-DB-on-2MB-OS guest is launched
but get much worse when it is paused, save/restored, migrated,
or hot-failed.  So even if B is only slightly faster than C,
if B is much slower than A, this is a problem.

Does that make sense?

Some suggestions:
1) If it is possible for an administrator to determine how many
   large pages (both 2MB and 1GB) were requested by each domain
   and how many are currently whole-vs-splintered, that would help.
2) We may need some form of memory defragmenter

> -----Original Message-----
> From: Wei Huang [mailto:wei.huang2@xxxxxxx]
> Sent: Thursday, March 19, 2009 12:52 PM
> To: Dan Magenheimer
> Cc: George Dunlap; xen-devel@xxxxxxxxxxxxxxxxxxx;
> keir.fraser@xxxxxxxxxxxxx; Tim Deegan
> Subject: Re: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support
> 
> 
> Dan,
> 
> Thanks for your comments. I am not sure about which 
> splintering overhead 
> you are referring to. I can think of three areas:
> 
> 1. splintering in page allocation
> In this case, Xen fails to allocate requested page order. So it falls 
> back to smaller pages to setup p2m table. The overhead is 
> O(guest_mem_size), which is a one-time deal.
> 
> 2. P2M splits large page into smaller pages
> This is one directional because we don't merge smaller pages to large 
> ones. The worst case is to split all guest large pages. So 
> overhead is 
> O(total_large_page_mem). In long run, the overhead will converge to 0 
> because it is one-directional. Note this overhead also covers 
> when PoD 
> feature is enabled.
> 
> 3. CPU splintering
> If CPU does not support 1GB page, it automatically does splintering 
> using smaller ones (such as 2MB). In this case, the overhead 
> is always 
> there. But 1) this only happens to a small number of old chips; 2) I 
> believe that it is still faster than 4K pages. CPUID (1gb feature and 
> 1gb TLB entries) can be used to detect and stop this problem, if we 
> don't really like it.
> 
> I agree on your concerns. Customers should have the right to 
> make their 
> own decision. But that require new feature is enabled in the first 
> place. For a lot of benchmarks, splintering overhead can be 
> offset with 
> benefits of huge pages. SPECJBB is a good example of using 
> large pages 
> (see Ben Serebrin's presentation in Xen Summit). With that 
> said, I agree 
> with the idea of adding a new option in guest configure file.
> 
> -Wei
> 
> 
> Dan Magenheimer wrote:
> > I'd like to reiterate my argument raised in a previous
> > discussion of hugepages:  Just because this CAN be made
> > to work, doesn't imply that it SHOULD be made to work.
> > Real users use larger pages in their OS for the sole
> > reason that they expect a performance improvement.
> > If it magically works, but works slow (and possibly
> > slower than if the OS had just used small pages to
> > start with), this is likely to lead to unsatisfied
> > customers, and perhaps allegations such as "Xen sucks
> > when running databases".
> > 
> > So, please, let's think this through before implementing
> > it just because we can.  At a minimum, an administrator
> > should be somehow warned if large pages are getting splintered.
> > 
> > And if its going in over my objection, please tie it to
> > a boot option that defaults off so administrator action
> > is required to allow silent splintering.
> > 
> > My two cents...
> > Dan
> > 
> >> -----Original Message-----
> >> From: Huang2, Wei [mailto:Wei.Huang2@xxxxxxx]
> >> Sent: Thursday, March 19, 2009 2:07 AM
> >> To: George Dunlap
> >> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; keir.fraser@xxxxxxxxxxxxx; 
> >> Tim Deegan
> >> Subject: RE: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support
> >>
> >>
> >> Here are patches using the middle approach. It handles 1GB 
> >> pages in PoD
> >> by remapping 1GB with 2MB pages & retry. I also added code for 1GB
> >> detection. Please comment.
> >>
> >> Thanks a lot,
> >>
> >> -Wei
> >>
> >> -----Original Message-----
> >> From: dunlapg@xxxxxxxxx [mailto:dunlapg@xxxxxxxxx] On 
> Behalf Of George
> >> Dunlap
> >> Sent: Wednesday, March 18, 2009 12:20 PM
> >> To: Huang2, Wei
> >> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; keir.fraser@xxxxxxxxxxxxx; 
> >> Tim Deegan
> >> Subject: Re: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support
> >>
> >> Thanks for doing this work, Wei -- especially all the 
> extra effort for
> >> the PoD integration.
> >>
> >> One question: How well would you say you've tested the PoD
> >> functionality?  Or to put it the other way, how much do I need to
> >> prioritize testing this before the 3.4 release?
> >>
> >> It wouldn't be a bad idea to do as you suggested, and break things
> >> into 2 meg pages for the PoD case.  In order to take the best
> >> advantage of this in a PoD scenario, you'd need to have a balloon
> >> driver that could allocate 1G of continuous *guest* p2m 
> space, which
> >> seems a bit optimistic at this point...
> >>
> >>  -George
> >>
> >> 2009/3/18 Huang2, Wei <Wei.Huang2@xxxxxxx>:
> >>> Current Xen supports 2MB super pages for NPT/EPT. The 
> >> attached patches
> >>> extend this feature to support 1GB pages. The PoD 
> >> (populate-on-demand)
> >>> introduced by George Dunlap made P2M modification harder. 
> I tried to
> >>> preserve existing PoD design by introducing a 1GB PoD cache list.
> >>>
> >>>
> >>>
> >>> Note that 1GB PoD can be dropped if we don't care about 
> 1GB when PoD
> >> is
> >>> enabled. In this case, we can just split 1GB PDPE into 512x2MB PDE
> >> entries
> >>> and grab pages from PoD super list. That can pretty much make
> >>> 1gb_p2m_pod.patch go away.
> >>>
> >>>
> >>>
> >>> Any comment/suggestion on design idea will be appreciated.
> >>>
> >>>
> >>>
> >>> Thanks,
> >>>
> >>>
> >>>
> >>> -Wei
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> The following is the description:
> >>>
> >>> === 1gb_tools.patch ===
> >>>
> >>> Extend existing setup_guest() function. Basically, it tries to
> >> allocate 1GB
> >>> pages whenever available. If this request fails, it falls 
> >> back to 2MB.
> >> If
> >>> both fail, then 4KB pages will be used.
> >>>
> >>>
> >>>
> >>> === 1gb_p2m.patch ===
> >>>
> >>> * p2m_next_level()
> >>>
> >>> Check PSE bit of L3 page table entry. If 1GB is found (PSE=1), we
> >> split 1GB
> >>> into 512 2MB pages.
> >>>
> >>>
> >>>
> >>> * p2m_set_entry()
> >>>
> >>> Configure the PSE bit of L3 P2M table if page order == 18 (1GB).
> >>>
> >>>
> >>>
> >>> * p2m_gfn_to_mfn()
> >>>
> >>> Add support for 1GB case when doing gfn to mfn 
> translation. When L3
> >> entry is
> >>> marked as POPULATE_ON_DEMAND, we call 2m_pod_demand_populate().
> >> Otherwise,
> >>> we do the regular address translation (gfn ==> mfn).
> >>>
> >>>
> >>>
> >>> * p2m_gfn_to_mfn_current()
> >>>
> >>> This is similar to p2m_gfn_to_mfn(). When L3 entry s marked as
> >>> POPULATE_ON_DEMAND, it demands a populate using
> >> p2m_pod_demand_populate().
> >>> Otherwise, it does a normal translation. 1GB page is taken into
> >>> consideration.
> >>>
> >>>
> >>>
> >>> * set_p2m_entry()
> >>>
> >>> Request 1GB page
> >>>
> >>>
> >>>
> >>> * audit_p2m()
> >>>
> >>> Support 1GB while auditing p2m table.
> >>>
> >>>
> >>>
> >>> * p2m_change_type_global()
> >>>
> >>> Deal with 1GB page when changing global page type.
> >>>
> >>>
> >>>
> >>> === 1gb_p2m_pod.patch ===
> >>>
> >>> * xen/include/asm-x86/p2m.h
> >>>
> >>> Minor change to deal with PoD. It separates super page 
> >> cache list into
> >> 2MB
> >>> and 1GB lists. Similarly, we record last gpfn of sweeping 
> >> for both 2MB
> >> and
> >>> 1GB.
> >>>
> >>>
> >>>
> >>> * p2m_pod_cache_add()
> >>>
> >>> Check page order and add 1GB super page into PoD 1GB cache list.
> >>>
> >>>
> >>>
> >>> * p2m_pod_cache_get()
> >>>
> >>> Grab a page from cache list. It tries to break 1GB page 
> into 512 2MB
> >> pages
> >>> if 2MB PoD list is empty. Similarly, 4KB can be requested 
> from super
> >> pages.
> >>> The breaking order is 2MB then 1GB.
> >>>
> >>>
> >>>
> >>> * p2m_pod_cache_target()
> >>>
> >>> This function is used to set PoD cache size. To increase 
> PoD target,
> >> we try
> >>> to allocate 1GB from xen domheap. If this fails, we try 
> 2MB. If both
> >> fail,
> >>> we try 4KB which is guaranteed to work.
> >>>
> >>>
> >>>
> >>> To decrease the target, we use a similar approach. We first try to
> >> free 1GB
> >>> pages from 1GB PoD cache list. If such request fails, we 
> try 2MB PoD
> >> cache
> >>> list. If both fail, we try 4KB list.
> >>>
> >>>
> >>>
> >>> * p2m_pod_zero_check_superpage_1gb()
> >>>
> >>> This adds a new function to check for 1GB page. This function is
> >> similar to
> >>> p2m_pod_zero_check_superpage_2mb().
> >>>
> >>>
> >>>
> >>> * p2m_pod_zero_check_superpage_1gb()
> >>>
> >>> We add a new function to sweep 1GB page from guest memory. 
> >> This is the
> >> same
> >>> as p2m_pod_zero_check_superpage_2mb().
> >>>
> >>>
> >>>
> >>> * p2m_pod_demand_populate()
> >>>
> >>> The trick of this function is to do remap_and_retry if
> >> p2m_pod_cache_get()
> >>> fails. When p2m_pod_get() fails, this function will 
> splits p2m table
> >> entry
> >>> into smaller ones (e.g. 1GB ==> 2MB or 2MB ==> 4KB). That can
> >> guarantee
> >>> populate demands always work.
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> _______________________________________________
> >>> Xen-devel mailing list
> >>> Xen-devel@xxxxxxxxxxxxxxxxxxx
> >>> http://lists.xensource.com/xen-devel
> >>>
> >>>
> >>
> > 
> 
>

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