Re: [Xen-devel] [PATCH v1 09/10] libxl: introduce libxl_retrieve_domain_configuration

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Wei Liu writes ("Re: [PATCH v1 09/10] libxl: introduce 
libxl_retrieve_domain_configuration"):
> On Thu, Jul 17, 2014 at 11:59:01AM +0100, Ian Campbell wrote:
> > Also is there is any possibility that any of the operations needed to
> > gather the updated configuration might take a long time.
> 
> I think reading from xenstore and filesystem can be slow, in the sense
> that if there's much contention on these two resources it could take
> seconds to finish operations.

For the purposes of libxl, slow functions are defined as follows
(libxl_internal.h):

 * "Slow" functions includes any that might block on a guest or an
 * external script.  More broadly, it includes any operations which
 * are sufficiently slow that an application might reasonably want to
 * initiate them, and then carry on doing something else, while the
 * operation completes.  That is, a "fast" function must be fast
 * enough that we do not mind blocking all other management operations
 * on the same host while it completes.
 *
 * There are certain primitive functions which make a libxl operation
 * necessarily "slow" for API reasons.  These are:
 *  - awaiting xenstore watches (although read-modify-write xenstore
 *    transactions are OK for fast functions)
 *  - spawning subprocesses
 *  - anything with a timeout

Note that xenstore accesses are explicitly excluded.

Ian.

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