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

On Tue, 2015-02-10 at 14:34 -0700, Mike Latimer wrote:
> 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.


What is "freemem-slack" for? It seems to have been added in 7010e9b7 but
the commit log makes no mention of it whatsoever. Was it originally just
supposed to be the delta between the host memory and dom0 memory at
start of day?

This seems to then change in a39b5bc64, to add an arbitrary caP which
seems to be working around an invalid configuration (dom0_mem +
autoballooning on).

Now that we autodetect the use of dom0_mem and set autoballooning
correctly perhaps we should just revert a39b5bc64?


> > 
> > 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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