Compression question / problem

Dave Cragg dcragg at lacscentre.co.uk
Wed Jan 12 16:51:36 CST 2005


On 12 Jan 2005, at 21:23, Alex Tweedly wrote:

> Frank D. Engel, Jr. wrote:
>
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>> In general, you can use your (client) bandwidth more effectively by 
>> downloading several files at a time, but 50 might be a bit much.  It 
>> would probably be ideal to create (for example) 5 files of 10 
>> pictures each, then download those 5 files in parallel.
>>
>> Or try with 4 files (2 of 13 images, 2 of 12), for example...
>>
>> And if the files can be distributed to several servers, even better 
>> (but don't do that unless there will be a LOT of simultaneous 
>> downloads from numerous clients -- otherwise it's not worth the 
>> expense, the gains would be minimal).
>
> As Frank says, approx. 4 files to be downloaded in parallel will give 
> you the shortest time to complete the transfer. Be sure to use "load 
> URL" on each to start the transfer, then "put URL" (or any other 
> similar technique). At 4 files, each one is only 125K (for 50 files of 
> 10K each) - increasing beyond that point is approaching the territory 
> where they are small enough to run into noticeable start-up overheads, 
> enough to defeat your purpose.
>
> Note this approach could be considered slightly anti-social if the 
> Internet connection is slow-ish and shared with other users - doing 4 
> transfers in parallel will allow you use a large part of the bandwidth 
> even if there are other users trying to get something done. If that 
> situation is possible, and if it's a 56K or slower connection, I'd 
> limit myself to 2 parallel streams.
>

Unless the files are on different servers, It won't be any faster to 
download in parallel using "load url". For requests to the same server, 
"load url" queues up requests, and when one completes, it starts the 
next one.

I'd guess downloading one large file could be anything from a little to 
a lot faster. If it's an ftp download, the overheads for each request 
are relatively high. And if the clients are using a high latency 
connection (such as a satellite), it's quite noticeable. Http has less 
overhead, but I'd still expect the single file to be faster.

Dave



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