On 09/17/2012 10:18 AM, Jan Kiszka wrote:
> On 2012-09-17 10:07, Gilles Chanteperdrix wrote:
>> On 09/17/2012 09:43 AM, Jan Kiszka wrote:
>>> On 2012-09-17 08:30, Gilles Chanteperdrix wrote:
>>>>
>>>> Hi,
>>>>
>>>> looking at x86 latencies, I found that what was taking long on my atom
>>>> was masking the fasteoi interrupts at IO-APIC level. So, I experimented
>>>> an idea: masking at LAPIC level instead of IO-APIC, by using the "task
>>>> priority" register. This seems to improve latencies on my atom:
>>>>
>>>> http://sisyphus.hd.free.fr/~gilles/core-3.4-latencies/atom.png
>>>>
>>>> This implies splitting the LAPIC vectors in a high priority and low
>>>> priority sets, the final implementation would use ipipe_enable_irqdesc
>>>> to detect a high priority domain, and change the vector at that time.
>>>>
>>>> This also improves the latencies on my old PIII with a VIA chipset, but
>>>> it generates spurious interrupts (I do not know if it really is a
>>>> matter, as handling a spurious interrupt is still faster than masking an
>>>> IO-APIC interrupt), the spurious interrupts in that case are a
>>>> documented behaviour of the LAPIC.
>>>>
>>>> Is there any interest in pursuing this idea, or are x86 with slow
>>>> IO-APIC the exception more than the rule, or having to split the vector
>>>> space appears too great a restriction?
>>>
>>> Line-based interrupts are legacy, of decreasing relevance for PCI
>>> devices - likely what we are primarily interesting in here - due to MSI.
>>
>> Even if I enable MSI, the kernel still uses these irqs for the
>> peripherals integrated to the chipset, such as the USB HCI, or ATA
>> driver (IOW, non PCI devices).
>
> Those are all PCI as well. And modern chipsets include variants of them
> with MSI(-X) support.
Here is what I get on my workstation:
$ grep fasteoi /proc/interrupts
9: 0 0 0 0 IO-APIC-fasteoi acpi
19: 0 0 0 0 IO-APIC-fasteoi ahci
23: 280150305 0 0 0 IO-APIC-fasteoi
ehci_hcd:usb5, ehci_hcd:usb6
It is an sandy bridge processor, it has less than 2 years.
What exactly do you mean by "modern" ?
--
Gilles.
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