Re: Lost events in older kernels

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Sat, May 22, 2010 at 12:27:15PM +0200, Henrik Rydberg wrote:
> Rafi Rubin wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > On 05/22/10 03:42, Dmitry Torokhov wrote:
> >> On Sat, May 22, 2010 at 03:06:07AM -0400, Rafi Rubin wrote:
> >>> I'm playing with a project with a 2.6.29 kernel, and the userspace application
> >>> seems to miss some events.  Is there a particular fix that improved the handling?
> >>>
> >>> Also tried catting the dev to a file while testing, and the dump also is missing
> >>> some events.
> >>>
> >> No "interesting" patches went into evdev for a few release now...
> >>
> >> Hm, could it be that event queue is overflowing before userspace gets a
> >> chance to empty it. What kind of event rate are we talking here?
> >>
> > 
> > Quite possibly.  It is a multitouch device and we know Henrik's been concerned
> > with the load for a while.
> > 
> > So to put some numbers behind his fears:
> > 
> > 146668 hid events processed
> > 24952 evdev events captured with a cat
> > 30 seconds (give or take).
> > 
> > This is for a mix of different numbers of fingers, but continuous use for those
> > 30 seconds.  And X was running and reading the dev node too.
> 
> Others have experienced this too. Mika has a patch for this, increasing the
> (kernel) evdev buffer quite a bit, from 64 to 256. I believe the reason it is
> not sent upstream is because it increases the footprint by 3 Kb. Perhaps a
> dynamic solution could work here.
> 

Yes, if input devices could hint handlers about their "packet size"
then evdev could size it's event queue accordingly. I'd say we need to
keep about 8 packets worth of data (number is pulled right out of my
behind ;) )...

-- 
Dmitry