[ros-users] slam_gmapping: extracting pose history

[René Wagner]

Hi Brian,

On Mon, 2010-05-17 at 08:31 -0700, Brian Gerkey wrote:
> Thanks for the patch; that approach sounds very promising.
> 
> But my casual testing (using logged data via
> gmapping/test/basic_stage_localization.launch) suggests that, with the
> patch applied, slam_gmapping consumes more CPU, rather than less.

With a small number of scans and a large particle set, that is expected
(see below).

> As it stands, the common processing profile of slam_gmapping is a
> relatively low nominal load with periodic surges up to 100% of a core
> (presumably when building the occupancy grid).
> 
> With the patch applied, slam_gmapping consumes a steady 100% of a
> core.   Maybe this is because many unnecessary maps are being built?

A map is built for each particle. This is the fundamental mechanism that
allows the algorithm to discard past laser scans as all history is
covered by the particle set of <map,pose,weight> triples (under certain
probabilistic assumptions). The map building has to be done for all
particles as weights may change and result in a different particle
becoming the best one.

> A constant-time update step is generally preferred, but not
> necessarily if the constant factor means that a core is always pegged.

Suppose you have N particles and K scans. Using the old configuration,
ScanMatcher::registerScan() would be called K times using the more
expensive m_generateMap==true code path. With the proposed changes
applied, this is done N times (AFAICT).

K increases over time. As K>N, the old approach will have to do more
work (and, as K becomes large, it will eventually run out of memory). 
I suspect the basic_stage_localization bag file is too short to cover
the K>N case. Maybe someone with a larger log file would like to
test this or share such a log file in bag format?

Cheers,

Rene

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Dipl.-Inf. René Wagner                     Junior Researcher
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