On 26.03.25 12:41, Markus Armbruster wrote:
Hanna Czenczek <hre...@redhat.com> writes:
On 26.03.25 06:38, Markus Armbruster wrote:
Hanna Czenczek <hre...@redhat.com> writes:
FUSE allows creating multiple request queues by "cloning" /dev/fuse FDs
(via open("/dev/fuse") + ioctl(FUSE_DEV_IOC_CLONE)).
We can use this to implement multi-threading.
Note that the interface presented here differs from the multi-queue
interface of virtio-blk: The latter maps virtqueues to iothreads, which
allows processing multiple virtqueues in a single iothread. The
equivalent (processing multiple FDs in a single iothread) would not make
sense for FUSE because those FDs are used in a round-robin fashion by
the FUSE kernel driver. Putting two of them into a single iothread will
just create a bottleneck.
Therefore, all we need is an array of iothreads, and we will create one
"queue" (FD) per thread.
[...]
Signed-off-by: Hanna Czenczek <hre...@redhat.com>
---
qapi/block-export.json | 8 +-
block/export/fuse.c | 214 +++++++++++++++++++++++++++++++++--------
2 files changed, 179 insertions(+), 43 deletions(-)
diff --git a/qapi/block-export.json b/qapi/block-export.json
index c783e01a53..0bdd5992eb 100644
--- a/qapi/block-export.json
+++ b/qapi/block-export.json
@@ -179,12 +179,18 @@
# mount the export with allow_other, and if that fails, try again
# without. (since 6.1; default: auto)
#
+# @iothreads: Enables multi-threading: Handle requests in each of the
+# given iothreads (instead of the block device's iothread, or the
+# export's "main" iothread).
When does "the block device's iothread" apply, and when "the export's
main iothread"?
Depends on where you set the iothread option.
Assuming QMP users need to know (see right below), can we trust they
understand which one applies when? If not, can we provide clues?
I don’t understand what exactly you mean, but which one applies when has
nothing to do with this option, but with the @iothread (and
@fixed-iothread) option(s) on BlockExportOptions, which do document this.
Is this something the QMP user needs to know?
I think so, because e.g. if you set iothread on the device and the export,
you’ll get a conflict. But if you set it there and set this option, you won’t.
This option will just override the device/export option.
Do we think the doc comment sufficient for QMP users to figure this out?
As for conflict, BlockExportOptions.iothread and
BlockExportOptions.fixed-iothread do.
As for overriding, I do think so. Do you not? I’m always open to
suggestions.
If not, can we provide clues?
In particular, do we think they can go from an export failure to the
setting @iothreads here? Perhaps the error message will guide them.
What is the message?
I don’t understand what failure you mean.
+# For this, the FUSE FD is duplicated so
+# there is one FD per iothread. (since 10.1)
Is the file descriptor duplication something the QMP user needs to know?
I found this technical detail interesting, i.e. how multiqueue is implemented
for FUSE. Compare virtio devices, for which we make it clear that virtqueues
are mapped to I/O threads (not just in documentation, but actually in option
naming). Is it something they must not know?
Interesting to whom?
Users of QMP? Then explaining it in the doc comment (and thus the QEMU
QMP Reference Manual) is proper.
Yes, QEMU users. I find this information interesting to users because
virtio explains how multiqueue works there (see IOThreadVirtQueueMapping
in virtio.json), and this explains that for FUSE exports, there are no
virt queues, but requests come from that FD, which explains implicitly
why this doesn’t use the IOThreadVirtQueueMapping type.
In fact, if anything, I would even expand on the explanation to say that
requests are generally distributed in a round-robin fashion across FUSE
FDs regardless of where they originate from, contrasting with
virtqueues, which are generally tied to vCPUs.
Hanna
Just developers? Then the doc comment is the wrong spot.
The QEMU QMP Reference Manual is for users of QMP. It's dense reading.
Information the users are not expected to need / understand makes that
worse.
Hanna
+#
# Since: 6.0
##
{ 'struct': 'BlockExportOptionsFuse',
'data': { 'mountpoint': 'str',
'*growable': 'bool',
- '*allow-other': 'FuseExportAllowOther' },
+ '*allow-other': 'FuseExportAllowOther',
+ '*iothreads': ['str'] },
'if': 'CONFIG_FUSE' }
##
[...]
