Re: [PATCH 14/15] fuse: Implement multi-threading

[Markus Armbruster <armbru@redhat.com>]

Hanna Czenczek <hreitz@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 <hreitz@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"?  Is this something the QMP user needs to know?


> +#                                 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?

> +#
>  # Since: 6.0
>  ##
>  { 'struct': 'BlockExportOptionsFuse',
>    'data': { 'mountpoint': 'str',
>              '*growable': 'bool',
> -            '*allow-other': 'FuseExportAllowOther' },
> +            '*allow-other': 'FuseExportAllowOther',
> +            '*iothreads': ['str'] },
>    'if': 'CONFIG_FUSE' }
>  
>  ##

[...]