[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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