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[Xen-devel] Re: Page fault is 4 times faster with XI shadow mechanism

To: zhu <vanbas.han@xxxxxxxxx>
Subject: [Xen-devel] Re: Page fault is 4 times faster with XI shadow mechanism
From: "Robert Phillips" <rsp.vi.xen@xxxxxxxxx>
Date: Sun, 2 Jul 2006 10:53:33 -0400
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Okay, now I understand your question!  It's a good question, too.

In XI, the idea is to have a pool of SPTIs all ready to go.  When a page needs to be shadowed, I simply pull a SPTI  off the list, zero its pages, and it is ready for use.  No further memory allocation is needed.  This is the critical path and I want it as short as possible.  (The zeroing is irksome.  I plan to hook the idle loop and try to keep SPTIs pre-zeroed.)

The shadow pages and backlink pages are pages that the XI code has reserved and taken for the domain.  The domain owns them for its lifetime.  It only needs them for use as shadow pages and backlink pages.  It has no other dynamic need for pages.

Since the pages are owned by the domain, they are not available for use by some other domain.  However it would be possible to leave them in the domheap rather than having the domain grab them all up front.  Then they might be available for use by other domains, and all domains could share a pool of shadow pages.  But that approach would only be helpful if domains can overcommit memory.

I think the idea of domains overcommitting memory and sharing pages is perilous since each domain's behavior then depends on the good behavior of other domains, and there is no mechanism for domains to apply backpressure to each other to reduce their memory use.

Regarding the backlink pages.  As you note, the common use of backlinks is to mark guest page tables as readonly.  (The less common uses are to mark all guest pages as readonly (for logging dirty pages during live migrate), to find large pages and divide them when they contain a guest page table, and to invalidate PTEs when a pfn-to-mfn mapping changes.)

One reason I have backlinks on all guest pages is because one can't know ahead of time which guest pages are (or will become) GPTs.  When the code first detects a guest page being used as a guest page table, it would have to do a linear search to find all SPTEs that point to the new guest page table, so it can mark them as readonly.

The backlink mechanism is particularly clean and simple precisely because there is a backlink per SPTE, regardless of whether the SPTE points to a GP, GPT, SPT or nothing.  This lets the backlinks be organized as an array with 512 elements, one to one with the SPTEs.  Given a backlink it's trivial to find the corresponding SPTE and vice-versa.

If one wanted to have backlinks for only the SPTEs that point to GPTs, things would really get complex.  The backlinks themselves would have to be organized and dynamically allocated and freed, and in the critical page fault path.

One could do without backlinks altogether if one were willing to put up with linear searching.  It's a space/performance tradeoff.  I think, with machines now having many megabytes of memory, users are more concerned about performance than a small memory overhead.

-- rsp

On 7/2/06, zhu < vanbas.han@xxxxxxxxx> wrote:

Robert Phillips 写道:
> Well I don't.  I simply pre-allocate a pool of SPTI's.  It can be quite a
> large pool but certainly not one-SPTI per MFN.  SPTIs are allocated on
> demand (when a guest page needs to be shadowed) and, when the pool runs
> low,
> the LRU SPTs are torn down and their SPTIs recycled.
Well what I mean is that we should not connect a snapshot page with a
SPTI at the first time the SPTIs are reserved. It would be better to
manage these snapshot pages in another dynamic pool.
BTW: What do you think of the backlink issue mentioned in my previous mail?
> Currently I allocate about 5% of system memory for this purpose (this
> includes the SPT, its snapshot and the backlink pages) and, with that
> reasonable investment, we get very good performance.  With more study, I'm
> sure things could be tuned even better.  (I hope I have properly understood
> your questions.)
> -- rsp
> On 7/1/06, zhu <vanbas.han@xxxxxxxxx > wrote:
>> Hi,
>> After taking some time to dig into your patch about XI Shadow page
>> table, I have to say it's really a good design and implementation IMHO,
>> especially the parts about the clear hierarchy for each smfn,decision
>> table and how to support 32nopae in a rather elegant way. However, I
>> have several questions to discuss with you.:-)
>> 1) It seems XI shadow pgt reserve all of the possible resources at the
>> early stage for HVM domain(the first time to create the asi). It could
>> be quite proper to reserve the smfns and sptis. However, do we really
>> need to reserve one snapshot page for each smfn at first and retain it
>> until the HVM domain is destroyed? I guess a large number of gpts will
>> not been modified frequently after them are totally set up. IMHO, it
>> would be better to manage these snapshot pages dynamic. Of course, this
>> will change the basic logistic of the code, e.g. you have to sync the
>> shadow pgt when invoke spti_make_shadow instead of leaving it out of
>> sync, you can't set up the total low level shadow pgt when invoke
>> resync_spte  since it could cost a lot of time.
>> 2) GP back link plays a very important role in XI shadow pgt. However,
>> it will also cause high memory pressure for the domain(2 pages for each
>> smfn). For these normal guest pages instead of GPT pages, I guess its
>> usage is limited. Only when invoke xi_invld_mfn, divide_large_page or
>> dirty logging, we will refer to the back link for these normal guest
>> pages. Is it reasonable to implement the back link only for the GPT
>> pages? Of course, this will increase the complexity of the code a little.
>> 3) Can you show us the statistics between the current shadow pgt and XI
>> pgt for some critical operations, such as shadow_resync_all, gva_to_gpa,
>> shadow_fault and so on. I'm really curious about it.
>> I have to say I'm not very familiar with the current shadow pgt
>> implementation so I could miss some important considerations when I post
>> these questions. Please point it out.
>> Thanks for sharing your idea and code with us. :-)
>> _______________________________________________________
>> Best Regards,
>> hanzhu
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel

Robert S. Phillips                          Virtual Iron Software
rphillips@xxxxxxxxxxxxxxx                Tower 1, Floor 2
978-849-1220                                 900 Chelmsford Street
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