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authorPeter Xu <peterx@redhat.com>2018-10-09 14:27:13 +0800
committerMarkus Armbruster <armbru@redhat.com>2018-12-12 09:55:57 +0100
commit9ab84470ffc2781df3acd4607bc6d2ae64d6d7e3 (patch)
tree0096dc70b9747e76ee7d144f057fcabe626ab235 /docs/interop
parent34f1f3e06d8824740d3bc41556f8300f0fb463cf (diff)
monitor: Suspend monitor instead dropping commands
When a QMP client sends in-band commands more quickly that we can process them, we can either queue them without limit (QUEUE), drop commands when the queue is full (DROP), or suspend receiving commands when the queue is full (SUSPEND). None of them is ideal: * QUEUE lets a misbehaving client make QEMU eat memory without bounds. Not such a hot idea. * With DROP, the client has to cope with dropped in-band commands. To inform the client, we send a COMMAND_DROPPED event then. The event is flawed by design in two ways: it's ambiguous (see commit d621cfe0a17), and it brings back the "eat memory without bounds" problem. * With SUSPEND, the client has to manage the flow of in-band commands to keep the monitor available for out-of-band commands. We currently DROP. Switch to SUSPEND. Managing the flow of in-band commands to keep the monitor available for out-of-band commands isn't really hard: just count the number of "outstanding" in-band commands (commands sent minus replies received), and if it exceeds the limit, hold back additional ones until it drops below the limit again. Note that we need to be careful pairing the suspend with a resume, or else the monitor will hang, possibly forever. And here since we need to make sure both: (1) popping request from the req queue, and (2) reading length of the req queue will be in the same critical section, we let the pop function take the corresponding queue lock when there is a request, then we release the lock from the caller. Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com> Signed-off-by: Peter Xu <peterx@redhat.com> Message-Id: <20181009062718.1914-2-peterx@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com>
Diffstat (limited to 'docs/interop')
-rw-r--r--docs/interop/qmp-spec.txt5
1 files changed, 3 insertions, 2 deletions
diff --git a/docs/interop/qmp-spec.txt b/docs/interop/qmp-spec.txt
index 8f7da0245d..67e44a8120 100644
--- a/docs/interop/qmp-spec.txt
+++ b/docs/interop/qmp-spec.txt
@@ -130,8 +130,9 @@ to pass "id" with out-of-band commands. Passing it with all commands
is recommended for clients that accept capability "oob".
If the client sends in-band commands faster than the server can
-execute them, the server will eventually drop commands to limit the
-queue length. The sever sends event COMMAND_DROPPED then.
+execute them, the server will stop reading the requests from the QMP
+channel until the request queue length is reduced to an acceptable
+range.
Only a few commands support out-of-band execution. The ones that do
have "allow-oob": true in output of query-qmp-schema.