[Fwd: Re: Compression question / problem]

Alex Tweedly alex at tweedly.net
Wed Jan 12 18:10:58 CST 2005


Chipp Walters wrote:

> Alex,
>
> Here's my math regarding the download time for a single 1 megapixel JPG
> file * 50. A Single 1024X768 JPEG compressed at Quality 60 is over 100K.
> Testing here it takes about 8 seconds on a cable modem to upload it to
> an FTP server. This doesn't take into account the handshake stuff as I'm
> using libURLSetStatusCallback to get feedback on when the file starts
> uploading and then is finished.
>
> So 8 seconds * 50 images = 400 seconds = 6 min 40 seconds. I really
> don't think waiting an extra 10 to 20 seconds makes *that* big a
> difference here-- and that is why I suggest making the decision based on
> other criteria. For instance, what happens if you lose your connection
> at 6 min 20 seconds? 

Your math is (of course) correct, and if we were talking about 6m 20sec 
vs 6m 40sec I would completely agree that separate files would be far 
better. But Richard said earlier that his 50 photos are highly 
compressed, and are around 10k each.

So in this situation, the trade-off is 10 seconds versus 20 seconds. The 
10 sec is an actual time for a single file download of 480K from own web 
server over 512K DSL - the 20 secs is extrapolated from 3 x 10k 
transfers from a web server on the east coast).

> Do you want your user to start all over? I
> personally would rather download each file independantly, keep track of
> them then have the user only download files not yet downloaded if the
> connection is broken. This is 'other criteria'.
>
> Also, as Dave mentions, libURL can't do 'parallel' downloads, so you
> don't get a speed boost you thought.

Yeah, I was a bit surprised by that.

So (for me) it would come down to time saving versus the potential cost 
of restarting big transfers.

At 100% time overhead and 10 second connection time - a single file 
would be "obvious"
At < 5% overhead and > 6min connection - separate files are equally 
"obvious"

Somewhere in between it might be a difficult choice ... though if I 
found I was in any doubt, I'd KISS and use separate files.

-- Alex.


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