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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