Avoid use of startwith in a large loop

[ben-grande]

Method 'startwith' adds considerable overhead.

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I know it's a cached value, but if >10ms is avoidable, I will take that.

ls-no-cache:2026-06-03T17:41:16.514792+00:00 dom0 qubesd[1566623]:     95895    0.013    0.000    0.013    0.000 {method 'startswith' of 'str' objects}

[marmarek]

Is your version faster?

[marmarek]

Specifically, do you see improvement in _load_usb_known_devices time?

[ben-grande]

Yes. I see improvements.

With startswith:

% grep startswith * | grep 95895
ls-no-cache:2026-06-03T17:41:16.514792+00:00 dom0 qubesd[1566623]:     95895    0.013    0.000    0.013    0.000 {method 'startswith' of 'str' objects}
ls-no-caching:2026-06-04T05:50:19.702238+00:00 dom0 qubesd[1864836]:     95895    0.010    0.000    0.010    0.000 {method 'startswith' of 'str' objects}
ls-no-caching.3:2026-06-04T06:19:08.386369+00:00 dom0 qubesd[1911131]:     95895    0.011    0.000    0.011    0.000 {method 'startswith' of 'str' objects}

% grep _load_usb_known_devices ls-no-cache ls-no-caching{,.3}
ls-no-cache:2026-06-03T17:41:16.514792+00:00 dom0 qubesd[1566623]:        56    0.039    0.001    0.069    0.001 core3ext.py:404(_load_usb_known_devices)
ls-no-caching:2026-06-04T05:50:19.702238+00:00 dom0 qubesd[1864836]:        56    0.027    0.000    0.049    0.001 core3ext.py:404(_load_usb_known_devices)
ls-no-caching.3:2026-06-04T06:19:08.386369+00:00 dom0 qubesd[1911131]:        56    0.035    0.001    0.067    0.001 core3ext.py:404(_load_usb_known_devices)

Without startswith:

% grep _load_usb_known_devices new/*
new/block-ls:2026-06-04T14:44:57.924217+00:00 dom0 qubesd[2571246]:        54    0.024    0.000    0.042    0.001 core3ext.py:404(_load_usb_known_devices)
new/usb-ls:2026-06-04T14:26:41.232351+00:00 dom0 qubesd[2542880]:        56    0.024    0.000    0.042    0.001 core3ext.py:404(_load_usb_known_devices)