Re: Lost events in older kernels

[Henrik Rydberg <rydberg@euromail.se>]

Dmitry Torokhov wrote:
> 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).

Thanks for these numbers, Rafi.

>>> 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 ;) )...
> 

Yeah. The bcm5974 driver sends 8 events per finger, plus some ST stuff. With
four fingers on the pad, that amounts to roughly 40 events per packet.

146668 / 24952 = 5.9
40 * 8 / 64 = 5.0

Looks reasonable, doesn't it?

Regarding the packet size hinting, the handler already knows which events are
potentially being sent, and could produce a reasonable buffer size from it. In
particular if it knows which events are bypassing filtering. ;-)

Henrik