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Re: [Qemu-devel] [PATCH v7 4/7] qapi: remove COMMAND_DROPPED event

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v7 4/7] qapi: remove COMMAND_DROPPED event
Date: Mon, 3 Sep 2018 09:30:52 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 09/03/2018 08:31 AM, Markus Armbruster wrote:


     client sends in-band command #1
     QEMU reads and queues
     QEMU dequeues in-band command #1
     in-band command #1 starts executing, but it's slooow
     client sends in-band command #2
     QEMU reads and queues
     client sends in-band command #8
     QEMU reads, queues and suspends the monitor
     client sends out-of-band command
--> time passes...
     in-band command #1 completes, QEMU sends reply
     QEMU dequeues in-band command #2, resumes the monitor
     in-band command #2 starts executing
     QEMU reads and executes out-of-band command
     out-of-band command completes, QEMU sends reply
     in-band command #2 completes, QEMU sends reply
     ... same for remaining in-band commands ...

The out-of-band command gets stuck behind the in-band commands.

The client can avoid this by managing the flow of in-band commands: have
no more than 7 in flight, so the monitor never gets suspended.

This is a potentially useful thing to do for clients, isn't it?

Eric, Daniel, is it something libvirt would do?

Right now, libvirt serializes commands - it never sends a QMP command until the previous command's response has been processed. But that may not help much, since libvirt does not send OOB commands.

I guess when we are designing what libvirt should do, and deciding WHEN it should send OOB commands, we have the luxury of designing libvirt to enforce how many in-flight in-band commands it will ever have pending at once (whether the current 'at most 1', or even if we make it more parallel to 'at most 7'), so that we can still be ensured that the OOB command will be processed without being stuck in the queue of suspended in-band commands. If we never send more than one in-band at a time, then it's not a concern how deep the qemu queue is; but if we do want libvirt to start parallel in-band commands, then you are right that having a way to learn the qemu queue depth is programmatically more precise than just guessing the maximum depth. But it's also hard to argue we need that complexity if we don't have an immediate use envisioned for it.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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