Re: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

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On Wed, 3 Jul 2019, Johannes Thumshirn wrote:

> On Wed, Jul 03, 2019 at 12:36:30AM +0200, Andrea Vai wrote:
> > On 02/07/19 13:51:17, Johannes Thumshirn wrote:
> > > On Tue, Jul 02, 2019 at 12:46:45PM +0200, Andrea Vai wrote:
> > > > Hi,
> > > >   I have a problem writing data to a USB pendrive, and it seems
> > > > kernel-related. With the help of Greg an Alan (thanks) and some
> > > > bisect, I found out the offending commit being
> > > > 
> > > > commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6
> > > >
> > > > [...]
> > > 
> > > Hi,
> > > 
> > > Can you please check what IO scheduler you have set for your USB pendrive?
> > > 
> > > i.e. with:
> > > cat /sys/block/$DISK/queue/scheduler
> > >
> > 
> > # cat /sys/block/sdf/queue/scheduler
> > [mq-deadline] none
> 
> One thing you can try as well is building a kernel with CONFIG_IOSCHED_BFQ and
> use it. Deadline is probably not the best choice for a slow drive.

Andrea, another thing you could try is to collect a usbmon trace under 
one of the "slow" kernels.  Follow the instructions in 
Documentation/usb/usbmon.txt.  I think you could kill the file-copy 
operation after just a couple of seconds; that should provide enough 
trace information to help see what causes the slowdown.

(If you want, do the same test with a "fast" kernel and then we'll 
compare the results.)

Alan Stern




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