On 04/27/2010 08:42 AM, Kevin Wolf wrote:
Am 27.04.2010 15:21, schrieb Anthony Liguori:
On 04/27/2010 08:18 AM, Kevin Wolf wrote:
The watermark is not some complex computed value, but actually the
statistic itself. We can get rid of handling a threshold in qemu by just
signalling "something has changed with this stat".
I'm really not arguing that qemu should do anything complex or even
define policy. It's just about avoiding polling all the time when
nothing has changed and polling too late when things are changing quickly.
Polling is really the right solution. It gives the management tool
ultimate flexibility in tweaking the heuristics as they see fit.
Isn't providing this flexibility completely orthogonal to polling vs.
event-based?
Except then we need to offer a generic statistics mechanism which seems
like it's going to add a fair bit of complexity. So far, the only
argument for it seems to be a misplaced notion that "polling is evil".
I'm not sure if "adding events is evil" is a much better position. :-)
The natural thing is really events here, because we want to get informed
every time something changes.
You want to be informed every time something has changed and the value
has met a boolean condition (value > threshold).
Polling is a workaround for cases where
you can't get these events. So I think it's you who should explain why
polling is so much better than using events.
Polling gives a management tool more flexibility in implementing the
evaluation condition.
Adding something to the protocol is a long term support statement. We
shouldn't add events unless we think they are events that we'll want to
support in the long term. If RHEV-M decides that instead of doing value
> threshold, they want to include additional information (maybe
factoring in I/O rate), then a new event needs to be added and we're
stuck supporting the old event forever.
Polling lets us avoid introducing new protocol operations as heuristics
change.
Note that IIUC the case is here different from the ballooning you
mentioned. The statistics for ballooning change all the time and you
don't want to get informed about changes but monitor the statistics all
the time, right?
No, generally speaking, you care about threshold conditions. For
instance, you want to know when guest reported free memory is < some
percentage of total memory. It's very similar really.
This is indeed a scenario where polling seems more
natural. But in contrast, the watermark usually doesn't change most of
the time and we want to know when changes happen.
Regards,
Anthony Liguori
Kevin
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