freemem-slack and large memory environments

[Mike Latimer <mlatimer@suse.com>]

Hi,

While testing commit 2563bca1, I found that libxl_get_free_memory returns 0 
until there is more free memory than required for freemem-slack. This means 
that during the domain creation process, freed memory is first set aside for 
freemem-slack, then marked as truly free for consumption.

On machines with large amounts of memory, freemem-slack can be very high (26GB 
on a 2TB test machine). If freeing this memory takes more time than allowed 
during domain startup, domain creation fails with ERROR_NOMEM. (Commit 
2563bca1 doesn't help here, as free_memkb remains 0 until freemem-slack is 
satisfied.)

There is already a 15% limit on the size of freemem-slack (commit a39b5bc6), 
but this does not take into consideration very large memory environments. 
(26GB is only 1.2% of 2TB), where this limit is not hit.

It seems that there are two approaches to resolve this:

 - Introduce a hard limit on freemem-slack to avoid unnecessarily large 
reservations
 - Increase the retry count during domain creation to ensure enough time is 
set aside for any cycles spent freeing memory for freemem-slack (on the test 
machine, doubling the retry count to 6 is the minimum required)

Which is the best approach (or did I miss something)?

Thanks!
Mike