Re: [Xen-devel] freemem-slack and large memory environments

[Mike Latimer <mlatimer@xxxxxxxx>]

On Monday, February 09, 2015 06:27:54 PM Mike Latimer wrote:
> 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)?

Sorry - forgot to CC relevant maintainers.

-Mike

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