TIFF performance: overviews and internal tiles: Surprise!

[Umberto Nicoletti]

How did you calculate the rendering times? Usually you should run the
program more than once and then average or even discard the lowest and
highest numbers for each run.
Suppose the following:
you run shp2img against the bare tiff once and the OS will do its best
to populate the filesystem cache if there is free memory. If you run
immediately the overview the fs cache will be already filled and the
OS will not be able to do any optimization, so the second run could
show a degraded performance.
To make sure that the initial conditions are the same for all maps you
could also reboot after each group run.

Umberto


On 9/2/07, Gregor Mosheh <gregor at hostgis.com> wrote:
> This test compares the effect of overviews and internal tiling on the
> performance of a TIFF raster data source.
>
> *** SETUP
>
> The data is USGS DOQQs (black-n-white) of San Francisco, California.
> Several DOQQs were downloaded and merged into a single GeoTIFF using
> gdal_merge.py.
>
> I then created three copies of the TIFF, with overviews, with internal
> tiling, and with both:
>     cp original.tif overviews.tif
>     gdaladdo overviews.tif 2 4 8 16 32
>     gdal_translate -co "TILED=YES" original.tif tiled.tif
>     gdal_translate -co "TILED=YES" original.tif tiledandoverviews.tif
>     gdaladdo tiledandoverviews.tif 2 4 8 16 32
>
> This set of overviews for this particular TIFF gives the following
> overview resolutions:
>     5984x21140, 2992x10570, 1496x5285, 748x2643, 374x1322
>
> All images use the same projection: EPSG 26910, aka UTM zone 10N with
> NAD83 datum.
> The spatial extent observed is:
>     543577.000 4150151.000 555545.000 4192431.000
>
> A mapfile is created specifying three layers, named BARE, OVERVIEW,
> TILE, BOTH, each one reading from the corresponding raster. No
> reprojection is being done in the mapfile.
>
>
> *** STORAGE SPACE
>
> original.tif            483 MB
> overviews.tif           646 MB
> tiled.tif               487 MB
> tiledandoverviews.tif   651 MB
>
> As was expected, adding overviews increases the file size by some 33%
> while internal tiling adds only 4-5 MB to the file size.
>
>
>
> *** RUNTIME SPEED
>
> # full view of the region
> shp2img -m mapfile.map -l BARE     -o bare-fullview.png
> Time: 0.484s
> shp2img -m mapfile.map -l OVERVIEW -o overview-fullview.png
> Time: 0.460s
> shp2img -m mapfile.map -l TILE     -o tile-fullview.png
> Time: 1.187s
> shp2img -m mapfile.map -l BOTH     -o both-fullview.png
> Time: 0.457s
>
> # 1 square kilometer pulled from the map
> shp2img -m mapfile.map -l BARE     -e 549561 4150150 550561 4151150 -o
> bare-1km.png
> Time: 1.072s
> shp2img -m mapfile.map -l OVERVIEW -e 549561 4150150 550561 4151150 -o
> overview-1km.png
> Time: 1.066s
> shp2img -m mapfile.map -l TILE     -e 549561 4150150 550561 4151150 -o
> tile-1km.png
> Time: 1.034s
> shp2img -m mapfile.map -l BOTH     -e 549561 4150150 550561 4151150 -o
> both-1km.png
> Time: 1.042s
>
> # a 3km square pulled from the map
> shp2img -m mapfile.map -l BARE     -e 547561 4149150 550561 4152150 -o
> bare-1km.png
> Time: 0.839s
> shp2img -m mapfile.map -l OVERVIEW -e 547561 4149150 550561 4152150 -o
> overview-1km.png
> Time: 0.822s
> shp2img -m mapfile.map -l TILE     -e 547561 4149150 550561 4152150 -o
> tile-1km.png
> Time: 0.840s
> shp2img -m mapfile.map -l BOTH     -e 547561 4149150 550561 4152150 -o
> both-1km.png
> Time: 0.825s
>
>
> At the full view, tiling actually hurt performance; presumably this was
> due to it seeking tiles and eventually not saving any time/seeks anyway.
>
> At the close-up views, I was quite surprised to see that tiles and
> overviews did indeed have an effect, but that the effect was only in the
> dozens of microseconds.
>
>
>
> I then repeated this experiment by fetching several more DOQQs and
> merging them into a 1 GB TIFF, then repeating the same generation and
> testing steps as above.
>
> * The tiling and overview size increase was basically the same: 4 MB
> fixed growth for tiles, and 33% growth for overviews.
>
> * The full views, all times remained in the same ratio, but increased by
> 50%. Not bad, considering that there was a 100% increase in file size.
>
> * For both the 1km and 2km extraction, the times were basically the same
> as for the 500 MB test above. The increased file size made a difference
> of 0.016s in the most dramatic case, which was that of the "bare"
> GeoTIFF. Other increases were 7 and 10ms.
>
>
> I then repeated again with a 2 GB GeoTIFF (just keep adding counties til
> I hit the limit, right?).
>
> Full view:
>    BARE       0.806s
>    OVERVIEW   0.759s
>    TILE       2.238s
>    BOTH       0.747s
> 2km square:
>    BARE       0.867s
>    OVERVIEW   0.832s
>    TILE       0.840s
>    BOTH       0.828s
>
> Again the same results!
>
>
> The conclusions are quite surprising:
>
> * The presence of overviews makes little difference in the performance
> otherwise, about 1/20 of a second in the most dramatic case.
>
> * Internal tiling seems likewise ineffectual, a difference of several
> microseconds.
>
> * However, internal tiling will hurt performance substantially in a case
> where the request ends up grabbing the entire image anyway.
>
>
> Is anybody else able to replicate these findings, or to achieve
> different results in a controlled experiment? Our hardware is rather
> beefy (8 CPU cores and a 4-disk RAID-5) so perhaps that's confounding my
> experiment? Perhaps these performance enhancers are mostly useful on a
> slower single-disk system?
>