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SubjectRe: [RFC PATCH 0/3] Weight-balanced binary tree + KVM growable memory slots using wbtree
On 02/24/2011 07:35 PM, Alex Williamson wrote:
> On Thu, 2011-02-24 at 12:06 +0200, Avi Kivity wrote:
> > On 02/23/2011 09:28 PM, Alex Williamson wrote:
> > > I had forgotten about<1M mem, so actually the slot configuration was:
> > >
> > > 0:<1M
> > > 1: 1M - 3.5G
> > > 2: 4G+
> > >
> > > I stacked the deck in favor of the static array (0: 4G+, 1: 1M-3.5G, 2:
> > > <1M), and got these kernbench results:
> > >
> > > base (stdev) reorder (stdev) wbtree (stdev)
> > > --------+-----------------+----------------+----------------+
> > > Elapsed | 42.809 (0.19) | 42.160 (0.22) | 42.305 (0.23) |
> > > User | 115.709 (0.22) | 114.358 (0.40) | 114.720 (0.31) |
> > > System | 41.605 (0.14) | 40.741 (0.22) | 40.924 (0.20) |
> > > %cpu | 366.9 (1.45) | 367.4 (1.17) | 367.6 (1.51) |
> > > context | 7272.3 (68.6) | 7248.1 (89.7) | 7249.5 (97.8) |
> > > sleeps | 14826.2 (110.6) | 14780.7 (86.9) | 14798.5 (63.0) |
> > >
> > > So, wbtree is only slightly behind reordering, and the standard
> > > deviation suggests the runs are mostly within the noise of each other.
> > > Thanks,
> >
> > Doesn't this indicate we should use reordering, instead of a new data
> > structure?
>
> The original problem that brought this on was scaling. The re-ordered
> array still has O(N) scaling while the tree should have ~O(logN) (note
> that it currently doesn't because it needs a compaction algorithm added
> after insert and remove). So yes, it's hard to beat the results of a
> test that hammers on the first couple entries of a sorted array, but I
> think the tree has better than current performance and more predictable
> when scaled performance.

Scaling doesn't matter, only actual performance. Even a guest with 512
slots would still hammer only on the first few slots, since these will
contain the bulk of memory.

> If we knew when we were searching for which type of data, it would
> perhaps be nice if we could use a sorted array for guest memory (since
> it's nicely bounded into a small number of large chunks), and a tree for
> mmio (where we expect the scaling to be a factor). Thanks,

We have three types of memory:

- RAM - a few large slots
- mapped mmio (for device assignment) - possible many small slots
- non-mapped mmio (for emulated devices) - no slots

The first two are handled in exactly the same way - they're just memory
slots. We expect a lot more hits into the RAM slots, since they're much
bigger. But by far the majority of faults will be for the third
category - mapped memory will be hit once per page, then handled by
hardware until Linux memory management does something about the page,
which should hopefully be rare (with device assignment, rare == never,
since those pages are pinned).

Therefore our optimization priorities should be
- complete miss into the slot list
- hit into the RAM slots
- hit into the other slots (trailing far behind)

Of course worst-case performance matters. For example, we might (not
sure) be searching the list with the mmu spinlock held.

I think we still have a bit to go before we can justify the new data
structure.

--
error compiling committee.c: too many arguments to function



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