Re: [Qemu-devel] [PATCH 3/4] docs: document how to configure the qcow2 L2/refcount caches

[Max Reitz <mreitz@redhat.com>]

On 05.06.2015 17:56, Alberto Garcia wrote:
> On Fri 05 Jun 2015 05:52:29 PM CEST, Max Reitz wrote:
>>> +Reducing the memory usage
>>> +-------------------------
>>> +It is possible to clean unused cache entries in order to reduce the
>>> +memory usage during periods of low I/O activity.
>>> +
>>> +The parameter "cache-clean-interval" defines an interval (in seconds)
>>> +after which all unused cache entries are removed from memory.
>>> +
>>> +This example removes all unused cache entries every 15 minutes:
>>> +
>>> +   -drive file=hd.qcow2,cache-clean-interval=900
>>> +
>>> +If unset, the default value for this parameter is 0 and it disables
>>> +this feature.
>>> +
>>> +Note that this functionality currently relies on the MADV_DONTNEED
>>> +argument for madvise() to actually free the memory, so it is not
>>> +useful in systems that don't follow that behavior.
>> I'd like a definition of what "unused" means (I know because of this
>> series, but I wouldn't know just from this document).
> It means that it hasn't been accessed in that interval (because it was
> not needed). Are you thinking of any other possible interpretation?

Well... To me, intuitively, "unused" entries in a cache are just empty 
entries. Just replacing one occurrence of "unused" by "not accessed 
during the past interval" or something like that would resolve the 
riddle for me. :-)

Max