Date:	Sun, 27 Feb 2011 11:54:29 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mtosatti@...hat.com, xiaoguangrong@...fujitsu.com
Subject: Re: [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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