Re: [PATCH v3] qapi/qmp: Add timestamps to qmp command responses

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Thu, Oct 13, 2022 at 05:00:26PM +0200, Markus Armbruster wrote:
>> Denis Plotnikov <den-plotnikov@yandex-team.ru> writes:
>> 
>> > Add "start" & "end" time values to qmp command responses.
>> 
>> Please spell it QMP.  More of the same below.
>> 
>> >
>> > These time values are added to let the qemu management layer get the exact
>> > command execution time without any other time variance which might be brought by
>> > other parts of management layer or qemu internals. This is particulary useful
>> > for the management layer logging for later problems resolving.
>> 
>> I'm still having difficulties seeing the value add over existing
>> tracepoints and logging.
>> 
>> Can you tell me about a problem you cracked (or could have cracked) with
>> the help of this?
>
> Consider your QMP client is logging all commands and replies in its
> own logfile (libvirt can do this). Having this start/end timestamps
> included means the QMP client log is self contained.

A QMP client can include client-side timestamps in its log.  What value
is being added by server-side timestamps?  According to the commit
message, it's for getting "the exact command execution time without any
other time variance which might be brought by other parts of management
layer or qemu internals."  Why is that useful?  In particular, why is
excluding network and QEMU queueing delays (inbound and outbound)
useful?

> Relying on tracing means that when a user attaches the QMP client log
> to a bug report, the timing info is missing. You have to ask the user
> to try to reproduce again with QEMU tracing enabled, which may be
> impossible, and then correlate the tracing output with the QMP client
> log.
>
> QEMU side tracing & logging is fine, but not a substitute for having
> this info included by default IMHO.

Not an answer to my question, but helpful all the same.