Re: Xen 4.2 Release Plan / TODO

[Ian Jackson <Ian.Jackson@eu.citrix.com>]

Dan Magenheimer writes ("RE: [Xen-devel] Xen 4.2 Release Plan / TODO"):
> After reading libxl.h, I'm not absolutely positive I understand
> all the conditions that would cause you to label a function as
> "slow" but I believe all the libxl_tmem_* functions are "fast".

Thanks for your reply.

There are a few operations that make a function necessarily have to be
slow in the libxl api sense.  These are: xenstore watches; spawning
subprocesses; anything with a timeout.

More broadly any function which is sufficiently slow that a caller
might reasonably want to initiate it, and then carry on doing
something else while the function completes.  So this includes any
operation which a toolstack might want to parallelise.

> All of them are strictly "call the hypervisor, wait for it to
> return" and none of the hypercalls (actually which are variations of
> the one tmem hypercall) require a callback to dom0 or to the
> calling guest... they all complete entirely inside the hypervisor.

Right, that sounds good.  I guess you also mean that this will always
be the case.

> Libxl_tmem_destroy may take a long time as it has to walk
> through and free some potentially very large data structures,
> but it is only used at domain destruction.

How long a time are we talking about ?  Would it be a scalability or
performance problem if an entire host's management toolstack had to
block, and no other management operations could be performed on any
domain for any reason, while the tmem destroy takes place ?

> Libxl_tmem_list does allocate some memory in userland that the
> hypercall fills synchronously (with ascii-formatted statistics/counters
> maintained entirely by the tmem code in the hypervisor).

Memory allocation in userland is fine.  I guess we're not talking
about megabytes here.

Ian.