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Re: Maximum QMP reply size
From: |
Daniel P . Berrangé |
Subject: |
Re: Maximum QMP reply size |
Date: |
Wed, 7 Sep 2022 08:57:49 +0100 |
User-agent: |
Mutt/2.2.6 (2022-06-05) |
On Tue, Sep 06, 2022 at 03:38:54PM -0400, John Snow wrote:
> Hi, I suspect I have asked this before, but I didn't write it down in
> a comment, so I forget my justification...
>
> In the QMP lib, we need to set a buffering limit for how big a QMP
> message can be -- In practice, I found that the largest possible
> response was the QAPI schema reply, and I set the code to this:
>
> # Maximum allowable size of read buffer
> _limit = (64 * 1024)
>
> However, I didn't document if this was a reasonable limit or just a
> "worksforme" one. I assume that there's no hard limit for the protocol
> or the implementation thereof in QEMU. Is there any kind of value here
> that would be more sensible than another?
As a reference, libvirt arbitrarily chose 10 MB as the QMP reply
limit. It is huge enough it'll be hard to make a QMP reply exceed
that, but also still tiny in the context of managing VMs on a host
with GB's of RAM. NB, this doesn't mean we allocate 10 MB every
time, it is just an upper bound - we only allocate what we actually
need.
With regards,
Daniel
--
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Re: Maximum QMP reply size, Daniel P . Berrangé, 2022/09/07
Re: Maximum QMP reply size,
Daniel P . Berrangé <=
Re: Maximum QMP reply size, Markus Armbruster, 2022/09/07