[mapguide-internals] RFC required for tile caching changes?

Trevor Wekel trevor.wekel at autodesk.com
Mon Jan 15 11:49:33 EST 2007 

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

+-----------------------------------------------------------------+
|Paul Spencer                          pspencer at dmsolutions.ca    |
+-----------------------------------------------------------------+
|Chief Technology Officer                                         |
|DM Solutions Group Inc                http://www.dmsolutions.ca/ |
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