[mapguide-internals] RFC required for tile caching changes?
Chris Claydon
chris.claydon at autodesk.com
Tue Jan 23 15:42:24 EST 2007
That should be relatively straightforward to achieve. I would add a
comment into serverconfig.ini indicating that any modifications to these
settings would:
a) Only take effect after a server restart.
b) Invalidate ALL existing cached tiles.
Chris.
_____
From: mapguide-internals-bounces at lists.osgeo.org
[mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Trevor
Wekel
Sent: January 23, 2007 1:34 PM
To: MapGuide Internals Mail List
Subject: RE: [mapguide-internals] RFC required for tile caching changes?
Hi Chris,
As an additional item, can we add serverconfig.ini parameters to control
the following:
- Size of the cache tile where the tiles assumed to be square "TileSize
= 300" for 300x300 tiles
- Tile block size, ie. the mod value in the discussion below
"TileBlockSize = 30"
The first item is good for tuning and is useful if you need to match
MapGuide tiles against an externally tiled raster source. The second
item could be used for tuning and eventually to support "block
rendering" of tiles.
Thanks,
Trevor
_____
From: mapguide-internals-bounces at lists.osgeo.org
[mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Chris
Claydon
Sent: Tuesday, January 23, 2007 1:26 PM
To: MapGuide Internals Mail List
Subject: RE: [mapguide-internals] RFC required for tile caching changes?
We seem to have digressed a little from my original question! I have
summarized the issues discussed in this thread below. I would like to
implement the changes as proposed in item 2), and to do so without the
need for an RFC. Please let me know if this is unacceptable for any
reason.
Thanks,
Chris.
1) We cannot conform directly with the Tile Map Service
Specification, because it does not currently support the concept of
grouping sets of tiles into individual folders. However, it would be
very simple to write a simple script or servlet to map from the
specification's path format directly to the MapGuide format.
2) Traian suggested a slightly modified folder naming convention
that provides an easier mapping to the original tile index. I like this
approach and would be happy to implement it this way. His proposal is as
follows:
"Another suggestion I have -- name the folder part of the path (30 *
tile index div 30) and then the file part would be (tile index mod 30).
So for tile 33,64 the folder would be:
Base Layer Group/30/60/3_4.png
And for tile -33,-64 the folder would be:
Base Layer Group/-30/-60/-3_-4.png
For tile 1,1, the folder would be:
Base Layer Group/0/0/1_1.png
For tile -1,-1 the folder would be:
Base Layer Group/-0/-0/-1_-1.png"
3) Various issues were brought up regarding how tiles are generated, and
how label placement can be optimized. These are beyond the scope of my
proposal, and can be addressed separately without being affected by the
changes I've proposed.
_____
From: mapguide-internals-bounces at lists.osgeo.org
[mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Robert
Bray
Sent: January 17, 2007 9:08 AM
To: MapGuide Internals Mail List
Subject: Re: [mapguide-internals] RFC required for tile caching changes?
Hey Paul, I think that is because you explicitly added him, for some
weird reason this list is already set to reply to list. The change in
question would not have made a difference in this case...
Bob
Paul Spencer wrote:
Steve's response bounced from the list, but fortunately reply-to-all was
still actually sending to everyone so I actually got the message ...
hint, hint ...
Paul
On 16-Jan-07, at 11:18 PM, Steve Lime wrote:
Paul: Er, I spent too much time at the Camp-to-Camp beer tap so my
memory is a bit fuzzy. We talked about several things from pre-computing
label positions to what you mention below. You are correct in how
MapServer works. It doesn't have any special "tiled" mode and hence
considers each tile just as any other map. In a single map context,
computing label positions post-clip makes sense.
Computing label positions pre-clip would result in a single, consistent
label position for each feature. For points and polygons that's
generally
OK I believe. For linear features the results might not always be
optimal
since the clipping often provides a nice means of segmenting long
features
so that get labeled multiple times. One could work around that at the
data level by segmenting long features ahead of time.
I don't believe it would hard to hack MapServer drawing routine
to do this and someone could test it out. Let me know if interested.
Steve
Paul Spencer <pspencer at dmsolutions.ca>
<mailto:pspencer at dmsolutions.ca> 01/15/07 7:03 PM >>>
Walt,
I don't believe it got documented. I spoke with Steve Lime (cc'd)
directly about this during OSGeo.
At the time, I believe Steve suggested label placement would be done
before features are clipped for rendering. I believe the way
mapserver works, it grabs all features that intersect a given bbox
then clips the features to the bbox then renders them. Labelling is
currently done post-clip. If it were done pre-clip, then labels
would be in a fixed position and could overlap tile boundaries
without problems.
I do believe that there were some potential problems with this
approach, but I don't know enough of mapserver internals to remember
what they were.
Steve, perhaps you could jump in and fix my explanation? Also, do
you think that label rendering changes would be a potential for
version 5?
Cheers
Paul
On 15-Jan-07, at 2:54 PM, Walt Welton-Lair wrote:
"In lieu of fixing this in MapServer (there is actually a proposal
to fix this)..."
Paul - is there per chance a public link to this proposal? I'd be
interested in looking at it.
Thanks,
Walt
-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org on behalf of
Paul
Spencer (External)
Sent: Mon 1/15/2007 7:40 PM
To: MapGuide Internals Mail List
Cc:
Subject: Re: [mapguide-internals] RFC required for tile caching
changes?
Trevor,
I don't think I got my idea across clearly :) Let me explain a
little more clearly.
In ka-Map, the size of a tile is configurable and by default is
200x200 px. There is a second concept that I've called a 'meta
tile'. A meta tile is some number of tiles wide x high,
typically 10
x 10. When a request for a tile comes to the tile server, it
checks
the cache.
If the tile exists, the tile (200x200px) is returned. This
happens
without invoking any map rendering ... the tile exists as an
image on
disk.
If the tile does not exist, then a map render happens. A map
the
size of the meta tile is rendered and sliced into tile-sized
chunks
that are stored on disk in the cache directory structure. While
the
tiles are being created, a lock file is used to 'pause' requests
for
tiles in the same meta-tile directory so that they can all be
served
from the same map draw when it is finished.
We chose this approach for several reasons. First, MapServer
suffers
from edge effects when rendering map images. This is normally
not
noticeable, but when you put several map draws together in a
tiled
client, it becomes horribly obvious. The solution was to render
a
slightly larger map image (about 10 pixels in fact) and clip out
the
center as the tile. Second, MapServer labelling only considers
the
current map draw. Depending on the size of the rendered label,
it
may not fit into a single tile. Depending on the size of a
feature,
the label may appear on several adjacent tiles. In lieu of
fixing
this in MapServer (there is actually a proposal to fix this),
rendering a much larger map and slicing it into smaller tiles
produces a reasonable effect. Finally, it is relatively
expensive to
parse the map file and find/read the data compared to rendering
so
rendering a single large image and slicing it is cheaper than
rendering each smaller tile (with diminishing returns).
I don't know how you actually render the tiles in MapGuide, but
I
don't think you suffer from the same edge and label issues.
The ka-Map directory structure looks something like this:
<map>/<layer-group>/<scale>/<row-of-metatile>/<col-of-metatile>/<row-
of-tile><col-of-tile>.<ext>
... which is similar to Chris' suggestion :) I don't get the
impression that you render the entire contents of the metatile
directory in one go though. Because I do, I only have to check
if
the meta-tile directory exists to know if any tile in the
directory
exists, which is a small optimizatin.
One difference between Chris' proposal and the way ka-Map works
is
that the scale and layer group are reversed. This makes it
theoretically possible to share the <layer-group> cache
directory
between different maps simply by creating symlinks or by
allowing a
per-layer-group setting that indicates the root directory to
keep the
tile cache in.
Cheers
Paul
On 15-Jan-07, at 11:49 AM, Trevor Wekel wrote:
> Hi everyone,
>
> I did some analysis on file systems a little while ago in
support
> of the
> directory structure change. Here are some approximate numbers
for
> directory entry and file system entries that I pulled from an
internet
> search:
>
> Linux ext3 - each file or directory is approx 8 bytes +
length of
> file
> or directory name
> Linux Reiserfs - each file or directory is 18 bytes + length
of
> file or
> directory name
>
> If we assume each file name has a format of R99999_C99999.PNG
then
> each
> file entry will require 26 bytes under ext3 and 34 bytes under
> ReiserFS.
>
> To facilitate fast directory access, tiles which are close to
each
> other
> should like in the same directory. This reduces the directory
entry
> reads for each tile request. If we also assume that the read
block
> size
> for the file system is 32kbytes (32/64k is often used in RAID
arrays)
> then a directory with 900 entries (30x30 block) will get read
in a
> single IOP (900*34 = 30600). And since the tiles are blocked
> together,
> it is very likely that adjacent tiles will fall within the
same
> directory. Since the directory was just read, it will be
cached
and
> there will be no disk access required.
>
> If we increase the size of the directories to more than 900
entries
> then
> we will incur more than IOP to read the bottom-most directory
> structure.
>
> I also think Traian's suggestion of Base Layer Group/
30/60/3_4.png is
> reasonable. It is more readable than the original mod scheme.
>
>> From a rendering perspective, dealing with 2000x2000 pixel
blocks
>> may be
> more efficient but cutting the tiles up from these larger
images
will
> require some computational effort. Rendering the tiles in the
same
> size
> as requested from the client reduces server load because the
HTTP
> request ends up being simply a file serving operation.
>
> With some of the latest optimizations I have been working on,
MapGuide
> tile serving speeds using "client-sized" tiles are quite
respectable.
> On a machine with a single 3GHz CPU, MapGuide can service more
than
> 100
> tiles per second assuming the tiles are being served from
memory,
> ie. no
> disk access. The additional computational effort required to
generate
> "client" tiles from larger pixel blocks will significantly
impact
> performance.
>
>
> Thanks,
> Trevor
>
> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org
> [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf
Of
Paul
> Spencer (External)
> Sent: Sunday, January 14, 2007 11:59 AM
> To: MapGuide Internals Mail List
> Subject: Re: [mapguide-internals] RFC required for tile
caching
> changes?
>
> FWIW, Traian's suggestion for naming folders based on a mod of
r/
c is
> essentially the same approach I took with ka-Map (for the same
reason
> that Chris brought this up) and it seems to work very well.
In ka-
> Map,
> tiles are rendered in blocks of about 2000x2000 pixels (tile
size is
> configurable but you might get 10 x 10 tiles in a typical
> configuration) and they would go in one of these sub-
directories. I
> think I figured out that this would be enough directories to
decently
> cache a pretty large area (say, all of the US) without running
into
> file
> system limits for most scales.
>
> It would be interesting to know the limits on:
>
> * files per directory
> * directories per directory
> * total number of directories
> * total number of files
> * minimum block(?) size in the file system compared to typical
size
> of a
> tile (wasted space per tile).
>
> for various operating systems/file system combinations. I
know
we use
> ReiserFS because it handles lots of small files better than
ext3
for
> instance. Knowing some of this might help make the caching
system
> more
> tunable in different environments.
>
> Paul
>
> On 13-Jan-07, at 11:09 AM, Traian Stanev wrote:
>
>> Can you also give an example for tiles with negative indices?
>> For example if in the current scheme the tile is -33, -64,
what
would
>> be the resulting directory path?
>>
>> R-2/C-3/-3_-4
>>
>> What about tile -1,-1
>>
>> Another suggestion I have -- name the folder part of the path
(30 *
>> tile index div 30) and then the file part would be (tile
index mod
>> 30).
>>
>> So for tile 33,64 the folder would be:
>>
>> Base Layer Group/30/60/3_4.png
>>
>> And for tile -33,-64 the folder would be:
>>
>> Base Layer Group/-30/-60/-3_-4.png
>>
>> For tile 1,1, the folder would be:
>>
>> Base Layer Group/0/0/1_1.png
>>
>> For tile -1,-1 the folder would be:
>>
>> Base Layer Group/-0/-0/-1_-1.png
>>
>>
>> It's a little weird around 0, but it allows for arbitrary
>> groupings of
>
>> tiles in folders (they don't have to be 30x30) since the tile
index
>> can be computed directly from the file path, by adding the
folder
>> term
>
>> to the file path term.
>>
>> Also, can you tell us what the problem is with having too
many
images
>> in one directory? Is it a file system issue? And how many
tiles is
>> the
>
>> limit, i.e. why is 30x30 the preferred grouping and not
1000x1000 for
>> example?
>>
>> Traian
>> -----Original Message-----
>> From: mapguide-internals-bounces at lists.osgeo.org on behalf of
Jason
>> Birch
>> Sent: Fri 1/12/2007 8:57 PM
>> To: MapGuide Internals Mail List
>> Cc:
>> Subject: RE: [mapguide-internals] RFC required for tile
caching
>> changes?
>>
>> Can I forward this to the "tiling" list for comment?
>>
>>
>> From: mapguide-internals-bounces at lists.osgeo.org on behalf of
Chris
>> Claydon
>> Sent: Fri 2007-01-12 12:24 PM
>> To: MapGuide Internals Mail List
>> Subject: RE: [mapguide-internals] RFC required for tile
caching
>> changes?
>>
>> A sample tile location would be as follows:
>>
>>
>> ......\
Repositories\TileCache\Samples_Sheboygan_Maps_Sheboygan\7
>> \Base
>> Layer Group\R2\C3\3_4.png
>>
>>
>> The R and C indices correspond to rows and columns where each
row
>> contains 30 tiles vertically and each column contains 30
tiles
>> horizontally.
>>
>>
>> This indicates that:
>>
>>
>> 1) We're at zoom level 7
>>
>> 2) We're in grouped row 2
>>
>> 3) We're in grouped column 3
>>
>> 4) The tile location within this grouped row/column
>> combination is (3,4).
>>
>>
>> The R and C indices never take a zero value (to avoid issues
with
>> negative values around the origin), so this tile corresponds
to a
>> location of (33, 64) in the old scheme.
>>
>>
>> I took a quick look at the Tile Map Service Specification,
but it
>> doesn't appear to support the concept of grouping blocks of
tiles
>> into subfolders in its current form. Can you provide more
>> information on whether this is possible?
>>
>>
>> Chris.
>>
>>
>> _______________________________________________
>> mapguide-internals mailing list
>> mapguide-internals at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/mapguide-internals
>
>
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> |Paul Spencer pspencer at dmsolutions.ca
|
>
+-----------------------------------------------------------------+
> |Chief Technology Officer
|
> |DM Solutions Group Inc
http://www.dmsolutions.ca/ |
>
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|DM Solutions Group Inc
http://www.dmsolutions.ca/ |
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