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SubjectPerformance halved with SP APIC: 2.3.40-7
Date
problem: performance gets roughly halved on a single processor box when
CONFIG_X86_UP_IOAPIC is enabled.

hardware: compaq deskpro. 400MHz PII/64Mb RAM/Intel PIIX4 controller, eepro100
network card.

Enabling APIC on single processor boxes seems to roughly halve the performance:
this isn't benchmark rates, but real world applications, e.g. the following
from /usr/bin/time:
2.3.42 without APIC
Command being timed: "bunzip2 tmp.tar.bz2"
User time (seconds): 17.86
System time (seconds): 0.21
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:18.09
Major (requiring I/O) page faults: 102
Minor (reclaiming a frame) page faults: 911
Page size (bytes): 4096
2.3.42 with APIC Command being timed: "bunzip2 tmp.tar.bz2"
User time (seconds): 37.80
System time (seconds): 0.42
Percent of CPU this job got: 196%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:19.49
Major (requiring I/O) page faults: 534
Minor (reclaiming a frame) page faults: 911

(bits snipped that didn't change). Note that the number of major page faults
has jumped massively (and the wall clock time was completely wrong). I've also
noticed that /proc/interrupts shows that the number of interrupts on IRQ 11
(usb-uhci & ethernet) seems much higher with APIC enabled.
Can't see any immediate fix, apart from a change to the config menu to put a
warning and maybe remove the comment "Such a kernel will still boot on
IO-APIC-less sstems with no slowdown at all."

I've got a feeling this is more to do with educating people like me who tend to
select 'y' for anything they've not tried before than actually being a bug but
it's probably worth clarifying.

cheers
john


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