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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" > in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html