Hello, I have a few questions about Preferences settings...
1. Memory: When computer is in use, use at most N % / When computer is not in use, use at most N%: Are these "N%" refers to total memory or free memory? I always see a much lower usage even if I set them very high. Does that mean the projects running just don't need that much?
2. Page/swap file: use at most N %: How does this even possible? I thought Windows will automatically determine if a program should swap to page file or not, based on whether the RAM is full and whether a program is active, or am I wrong? Also if I'm using a SSD, should I set this to 0 to prevent to much writes? Is there any side effect to disable this?
3. Network: I notice that BOINC only download at most 2 files at the same time, even if I don't set any limitations, is it possible to override this behavior? Also, is it possible to use more connections per file?
Thank you!

1. Memory: When computer is in use, use at most N % / When computer is not in use, use at most N%: Are these "N%" refers to total memory or free memory? I always see a much lower usage even if I set them very high. Does that mean the projects running just don't need that much?

The values are for the total amount of RAM that BOINC plus science applications is allowed to use.

2. Page/swap file: use at most N %: How does this even possible? I thought Windows will automatically determine if a program should swap to page file or not, based on whether the RAM is full and whether a program is active, or am I wrong?

Windows always uses the page file, ever since Vista. Even if you have 1TB of RAM in the machine, it'll try to use as much as page file. With this value you tell BOINC how much of the page file usage it can use.

Also if I'm using a SSD, should I set this to 0 to prevent to much writes? Is there any side effect to disable this?

There shouldn't be any side-effects if you do not use the "Leave non-GPU tasks in memory while suspended" preference. As for SSDs, Windows writes more to disk every minute than BOINC does, and aside from that, the latest SSDs are rated for millions of writes, more than you can make full in its lifetime.

3. Network: I notice that BOINC only download at most 2 files at the same time, even if I don't set any limitations, is it possible to override this behavior? Also, is it possible to use more connections per file?

Yes, that's possible through the options section in the cc_config.xml file:

<cc_config>
   <log_flags>
   </log_flags>
   <options>
      <max_file_xfers>N</max_file_xfers>
      <max_file_xfers_per_project>N</max_file_xfers_per_project>
   </options>
</cc_config>

<max_file_xfers>N</max_file_xfers>
Maximum number of simultaneous file transfers (default 8).
<max_file_xfers_per_project>N</max_file_xfers_per_project>
Maximum number of simultaneous file transfers per project (default 2).

You can easily make a fully populated cc_config.xml file by opening BOINC Manager->View->Advanced view->Options->Event Log options...->uncheck file_xfer->OK. This writes a complete cc_config.xml file to your data directory.
Reopen the Event Log options and recheck file_xfer->OK as that's something you do want.
Next open cc_config.xml in Notepad and find the lines above, then change their value to something new. Use Notepad->File->Save to save changes.
Then open BOINC Manager again->Options->Read config files.

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1. Memory: When computer is in use, use at most N % / When computer is not in use, use at most N%: Are these "N%" refers to total memory or free memory? I always see a much lower usage even if I set them very high. Does that mean the projects running just don't need that much?

The values are for the total amount of RAM that BOINC plus science applications is allowed to use.

2. Page/swap file: use at most N %: How does this even possible? I thought Windows will automatically determine if a program should swap to page file or not, based on whether the RAM is full and whether a program is active, or am I wrong?

Windows always uses the page file, ever since Vista. Even if you have 1TB of RAM in the machine, it'll try to use as much as page file. With this value you tell BOINC how much of the page file usage it can use.

Also if I'm using a SSD, should I set this to 0 to prevent to much writes? Is there any side effect to disable this?

There shouldn't be any side-effects if you do not use the "Leave non-GPU tasks in memory while suspended" preference. As for SSDs, Windows writes more to disk every minute than BOINC does, and aside from that, the latest SSDs are rated for millions of writes, more than you can make full in its lifetime.

3. Network: I notice that BOINC only download at most 2 files at the same time, even if I don't set any limitations, is it possible to override this behavior? Also, is it possible to use more connections per file?

Yes, that's possible through the options section in the cc_config.xml file:

<cc_config>
   <log_flags>
   </log_flags>
   <options>
      <max_file_xfers>N</max_file_xfers>
      <max_file_xfers_per_project>N</max_file_xfers_per_project>
   </options>
</cc_config>

<max_file_xfers>N</max_file_xfers>
Maximum number of simultaneous file transfers (default 8).
<max_file_xfers_per_project>N</max_file_xfers_per_project>
Maximum number of simultaneous file transfers per project (default 2).

You can easily make a fully populated cc_config.xml file by opening BOINC Manager->View->Advanced view->Options->Event Log options...->uncheck file_xfer->OK. This writes a complete cc_config.xml file to your data directory.
Reopen the Event Log options and recheck file_xfer->OK as that's something you do want.
Next open cc_config.xml in Notepad and find the lines above, then change their value to something new. Use Notepad->File->Save to save changes.
Then open BOINC Manager again->Options->Read config files.

Thank you very much, but I think you didn't understand the meaning of the 1st question.
I want to know where do the N% calculated from, the total memory or free memory? For example, I have 4GB physical memory, but only 3GB are free, and I set the value to 50%, so will BOINC use 1.5GB or 2GB?

The value you set is the maximum amount of RAM BOINC can use at those times, whether it'll use that is up to the project's science applications. I think there's only one or two still left that have huge tasks using 1GB of memory, but the majority uses way less than that and so the maximum is never reached.

By the way, was it necessary to quote my whole post?

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Thank you, but is the maximum calculated by the total amount or the free memory?

Total memory. Side note, on 32-bit systems that can be significantly less than physically installed.

Thank you.