Re: [PATCH v6 00/12] *** A Method for evaluating dirty page rate ***

From: Zheng Chuan <zhengchuan@huawei.com>
To: David Edmondson <dme@dme.org>, <quintela@redhat.com>,
	<eblake@redhat.com>,  <dgilbert@redhat.com>,
	<berrange@redhat.com>
Cc: alex.chen@huawei.com, ann.zhuangyanying@huawei.com,
	zhang.zhanghailiang@huawei.com, xiexiangyou@huawei.com,
	qemu-devel@nongnu.org
Subject: Re: [PATCH v6 00/12] *** A Method for evaluating dirty page rate ***
Date: Mon, 31 Aug 2020 17:55:39 +0800	[thread overview]
Message-ID: <b597768c-eb89-d131-ebf4-5a82bcf1e326@huawei.com> (raw)
In-Reply-To: <m2h7sjchid.fsf@dme.org>

On 2020/8/31 17:05, David Edmondson wrote:
> Trying to think like a control plane developer and user (of which I am
> neither) raised some questions about the overall interface provided
> here. If everyone else is happy with the current interface, then I'll
> shut up :-)
> 
> It seems like it should be possible to query the last measured dirty
> rate at any time. In particular, it should be possible to query the
> value before any rate has been measured (either returning an error, or
> if that is unpalatable perhaps a result with a zero interval to indicate
> "this data isn't useful"), but also *during* a subsequent measurement
> period.
> 
Hi, Thank you for your review.

For now,
i. if we query the value before any rate has been measured, it will return unstarted,
   and dirtyrate will return -1.
{"return":{"status":"unstarted","dirty-rate":-1,"start-time":0,"calc-time":0},"id":"libvirt-14"}

ii.if we specify the measurement interval like -1 or 61, it will return error
{"id":"libvirt-13","error":{"class":"GenericError","desc":"calc-time is out of range[1, 60]."}}

iii. We can query the last measured dirty rate at any time now as you expected in last patch version
     with returning the measurement timestamp and calc-time.

If i have missed some other scenes, please let me know:)

> That is, the result of the previous measurement should always be
> available on demand and a measurement becomes "current" when it
> completes.
> 
> Given that we allow the caller to specify the measurement interval, some
> callers might specify a long period. As only one measurement can be
> taken at a time, a long running measurement rules out taking a short
> measurement. That's probably okay, but does lead me to wonder whether
> the API should include a mechanism allowing the cancellation of an
> in-progress measurement.

That's why we restrict the maximum time to 60s, i think this is enough and also
not too long for user to observe the average dirty rate.
I think it is may a little expensive and hardly used to implement cancellation mechanism.

On the other hand, users could call several times with the measurement interval
like 1s, to improve its accuracy otherwise observe a long time.

> 
> dme.
> 