Limit number of requests for pre-fetching

[kaymes]

I noticed some ocational buffer under-runs as a consequence of PR #393 for which I made a fix here.

The issue is, that the pre-fetch algorithm can potentially fire of lots of small requests to keep the amount of pending bytes at the desired level. If bandwidth limits are hit, this leads to responses being interleaved - all requests are served simultaneously, which means the total data rate is great but each single request experiences a low data rate. A consequence can be, that the first request isn't responded to in time and we get a buffer under-run.

I solved this by limiting the amount of open requests when pre-fetching: a pre-fetch request is only sent when less than 4 requests are open. This leads to less requests which are larger. Thus, individual requests should still get decent download rates.

I experimented with different values. Using a limit of 3 leads to significantly lower overall download rates. Thus, I chose 4.

I also increased the amount of data that is requested ahead of the current read positions.

So far I haven't had any more buffer under-runs with these changes. Fingers crossed.

[kaymes]

This PR has been sitting here for two weeks without getting merged or commented on. I'm just wondering whether there's anything I need to do to get this processed. Or is everyone on vacation already? :-)

[sashahilton00]

I've been pretty busy for the past few weeks, I guess the other maintainers have as well. Anyway, just skimmed over it, did a quick test, seems fine, merged.

[kaymes]

Thank you.