xmodmap really slow

[Alan Hourihane]

On 30/11/2018 09:38, Alan Hourihane wrote:
> On 30/07/2018 23:25, Alan Hourihane wrote:
>> On 30/07/18 22:00, Adam Jackson wrote:
>>> On Thu, 2018-07-26 at 12:25 +0100, Alan Hourihane wrote:
>>>> Anyone tried just doing this recently....
>>>>
>>>> xmodmap -pke > /tmp/keydump
>>>> xmodmap /tmp/keydump
>>>>
>>>> And watch the Xserver hang for quite some time.
>>> Seems to be nearly instantaneous against an Xvfb, but did cause a
>>> noticable hiccup with Xorg. Haven't investigated more closely, so I'd
>>> suspect (in rough order of likelihood):
>>>
>>> a) something super-linear in the number of input devices
>>> b) some weird slow ioctl syncing the X state to the kernel
>>> c) somehow you end up on the "lets spawn xkbcomp" path and the
>>> fork/exec overhead kills you
>>>
>> Definitely not the last, moving "xkbcomp" from /usr/bin results in the
>> same issue.
> Just checking.....
>
> Has anyone looked into this issue ?

Running perf top while it's happening shows this....

    26.77%  Xorg           [.] ResourceClientBits
    15.09%  Xorg           [.] GrabMatchesSecond
    12.62%  Xorg           [.] DetailSupersedesSecond.isra.1
    12.55%  Xorg           [.] DeletePassiveGrabFromList
     8.83%  Xorg           [.] GrabSupersedesSecond
     5.55%  Xorg           [.] AddPassiveGrabToList
     3.09%  Xorg           [.] DeletePassiveGrab
     2.00%  Xorg           [.] xi2mask_merge
     1.16%  libc-2.27.so   [.] _int_malloc
     0.92%  libc-2.27.so   [.] _int_free
     0.59%  Xorg           [.] XkbSendMap

Xorg is totally frozen while this is happening. No rendering occurs at all.

Alan.