Of course a workaround could be to start 3 Boinc instances at the same time, each for one project and there I could limit the threads and remove the app_config.xml. But I was hoping for an easier way.

Is that on an AMD?

If a project wrongly under estimates the duration of tasks then there is little that BOINC can do to stop that stop that project from flooding you with work. If it is consistently happening then you should take it up with the project concerned.

Here are the details of what these settings do. As an example, assume the first setting was 2.0 days and the second (extra days) setting was 1.5 days. BOINC would regard a 'full' cache would be when the estimates of all tasks add up to > 3.5 days. As you complete and return work, BOINC will not take any action to replenish the cache until the work on hand falls below 2 days. So if you put a value in the extra days setting you create a hysteresis effect between a 'hi water' mark and a 'lo water' mark. For this example those marks are separated by 1.5 days. This is precisely why there would be long periods where no work is being requested.

If a user really wanted the best chance to have a stable level of work on hand, with regular top-ups, I think the best option is to put what is wanted in the first setting and leave the second one at zero. Please realise that BOINC can only make decisions about work fetch based on estimates.

You mention tasks for a single project taking between 5 mins and 10 days. Do these tasks come with proper estimates when you first receive them? No problem if they do, since BOINC can handle that. If they don't, you should complain bitterly in the project's forums since this is really bad behaviour which will cause lots of problems for your client when managing the work flow for multiple projects.

Assuming you are running projects that take minutes to hours to run then it will have very little impact on scheduling.

The second one is confusingly named "store additional x days", in reality this determines how often you do a call for work from each project, the lower the value the more frequent the check is made to see if you need work, and it can be a very small number (fraction of a day, 0.1 is about 2.4 hour intervals).

Also of course many projects have periods of time when there is no work available for a whole range of reasons, and this can really upset inter-project scheduling, particularly when one considers BOINC works on a medium/long term deficit basis to determine what projects get work and what projects are "ignored".

If I set <rec_half_life_days>X</rec_half_life_days> to 30 days would that improve scheduling or maybe setting it to the other extreme 1, help?

Would setting it to 0 force the scheduler to request work hourly and ignore the lengths of past work units?

Is there any other mechanism to speed scheduling?

There are many projects that force down work and flood the cache, ignoring their resource share.

Projects servers can't actually 'force down work' - the firewall and NAT (Network Address Translation) in your router would prevent that.

Project servers can only communicate with your computer in the form of a reply to a request made by your computer.

I'd be interested to see the request/reply pair for an event like this - the <sched_op_debug> event log flag is useful for this:

11/02/2019 08:45:02 | SETI@home | Sending scheduler request: To report completed tasks.
11/02/2019 08:45:02 | SETI@home | Reporting 5 completed tasks
11/02/2019 08:45:02 | SETI@home | Requesting new tasks for NVIDIA GPU
11/02/2019 08:45:02 | SETI@home | [sched_op] NVIDIA GPU work request: 7683.10 seconds; 0.00 devices
11/02/2019 08:45:06 | SETI@home | Scheduler request completed: got 8 new tasks
11/02/2019 08:45:06 | SETI@home | [sched_op] estimated total NVIDIA GPU task duration: 7907 seconds

Note that the amounts requested and received are expressed in seconds - duration requested and runtime estimated. They should match.

To find the fraction that each project is using, you add them all up and then divide by each project's resource share.
For example, if you have three projects, all set to 100%, then each project gets 33% of the time (or actually cores).

Further details of my attempts to recover should be handled in PM's with the appropriate wiki manager.

In that case, PM me. To check if you have an account there, I require an email or IP address. If there wasn't an account to begin with, I can make one for you. Please make sure that you're talking about the correct Wiki, we have two.

What happened to my account and how can I get it back?

That depends, what do you think the user name is? It isn't marmot, I checked that already, there is no such name. And seeing that all accounts starting from 2011 onwards are still there, yours -if you had one- must still be there as well. Try different email addresses.

It could be categorized under retired projects, if the category exists, as this Wiki article still shows up from Google search hits when searching for information on the project.?

Whether I set it to a separate page about it being retired or not, that won't matter in this as Google will still be able to find it and all of the other project pages in there. I'll send an email to the PMC to ask them what they want to do with these pages, delete or just easier put a link back into the visible pages and have people add to/adapt these pages.

As an aside, if Malariacontrol.net is no more it seems strange that it is one of the few projects on the recently announced BOINC@TOKYO site !

As far as I know, the BOINC Wikis don't purge accounts based on inactivity. You can't even make an account there, that always has to run via David due to spammers.
We also don't actively link to any projects from the user manual wiki, that's all done via the BOINC front page and the add-project wizard. Those pages may still be there, but if all is well, you cannot get there from a link in the index of https://boinc.berkeley.edu/wiki/User_manual or any of its sub-links.