On Tue, 2016-05-24 at 08:53 +0200, Hans de Goede wrote:
> Hi,
>
> On 23-05-16 19:36, James Bottomley wrote:
> > On Mon, 2016-05-23 at 13:49 +0200, Hans de Goede wrote:
> > > Commit 198de51dbc34 ("USB: uas: Limit qdepth at the scsi-host
> > > level")
> > > removed the scsi_change_queue_depth() call from
> > > uas_slave_configure() assuming that the slave would inherit the
> > > host's queue_depth, which that commit sets to the same value.
> > >
> > > This is incorrect, without the scsi_change_queue_depth() call the
> > > slave's queue_depth defaults to 1, introducing a performance
> > > regression.
> > >
> > > This commit restores the call, fixing the performance regression.
> > >
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Fixes: 198de51dbc34 ("USB: uas: Limit qdepth at the scsi-host
> > > level")
> > > Reported-by: Tom Yan <tom.ty89@xxxxxxxxx>
> > > Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
> > > ---
> > > drivers/usb/storage/uas.c | 1 +
> > > 1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/usb/storage/uas.c
> > > b/drivers/usb/storage/uas.c
> > > index 16bc679..ecc7d4b 100644
> > > --- a/drivers/usb/storage/uas.c
> > > +++ b/drivers/usb/storage/uas.c
> > > @@ -835,6 +835,7 @@ static int uas_slave_configure(struct
> > > scsi_device
> > > *sdev)
> > > if (devinfo->flags & US_FL_BROKEN_FUA)
> > > sdev->broken_fua = 1;
> > >
> > > + scsi_change_queue_depth(sdev, devinfo->qdepth - 2);
> >
> > Are you sure about this? For spinning rust, experiments imply that
> > the optimal queue depth per device is somewhere between 2 and 4.
> > Obviously that's not true for SSDs, so it depends on your use
> > case. Plus, for ATA NCQ devices (which I believe most UAS is
> > bridged to) you have a maximum NCQ depth of 31.
>
> So this value is the same as host.can_queue, and is what uas has
> always used, basically this says it is ok to queue as much as the
> bridge can handle. We've seen a few rare multi-lun devices, but
> typically almost all uas devices have one lun, what I really want to
> do here is give a maximum and let say the sd driver lower that if it
> is sub-optimal.
If that's what you actually want, you should be setting sdev
->max_queue_depth and .track_queue_depth = 1 in the template.
You might also need to add calls to scsi_track_queue_full() but only if
the devices aren't responding QUEUE_FULL correctly.
James
> Also notice that uas is used a lot with ssd-s, that is mostly what
> I want to optimize for, but it is definitely also used with spinning
> rust.
>
> And yes almost all uas devices are bridged sata devices (this may
> change in the near future though, with ssd-s specifically designed
> for usb-3 attachment, although sofar these all seem to use an
> embbeded sata bridge), so from this pov an upper limit of 31 makes
> sense, I guess, but I've not seen any bridges which actually do more
> then 32 streams anyways.
>
> Still this is a bug-fix patch, essentially a partial revert, to
> address performance regressions, so lets get this out as is and take
> our time to come up with some tweaks (if necessary) for the say 4.8.
>
> > There's a good reason why you don't want a queue deeper than you
> > can handle: it tends to interfere with writeback because you build
> > up a lot of pending I/O in the queue which can't be issued (it's
> > very similar to why bufferbloat is a problem in networks). In
> > theory, as long as your devices return the correct indicator
> > (QUEUE_FULL status), we'll handle most of this in the mid-layer by
> > plugging the block queue, but given what I've seen from UAS
> > devices, that's less than probable.
>
> So any smart ideas how to be nicer to spinning rust, without
> negatively impacting ssd-s? As said if I've to choice I think we
> should chose optimizing ssd-s, as that is where uas is used a lot
> (although usb attached harddisks are switching over to it too).
>
> Note I just checked the 1TB sata/ahci harddisk in my workstation and
> it is using a queue_depth of 31 too, so this really does seem like a
> mid-layer problem to me.
>
> Regards,
>
> Hans
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