On Wed, 2006-Sep-27 10:32:49 -0600, Scott Long wrote: >My theory here is that something in the kernel, likely VM/VFS, is >holding the Giant lock for an inordinate amount of time.
In the past (RELENG_5) I've had major problems with syncer delaying interrupt threads for long periods (I've seen 8msec). See http://lists.freebsd.org/pipermail/freebsd-stable/2005-February/012346.html I'm not sure if this is still a problem (but I am still having some problems which may be caused by excessive interrupt and will be doing some debugging as I get time). >I have a few ideas on tracking down the root cause, but they are pretty >pretty painful and slow. In my case, I was fairly certain that the problem I was seeing was excessive interrupt latency for my driver. The approach I took was to capture TSC, IRQ number and curproc address in lapic_handle_intr(), atpic_handle_intr() and at the beginning of my interrupt handler into a ring buffer. I'd dump the ring buffer into a file using a userland tool and then post-process the file looking for oddities. In my case, there was a _very_ high correlation between long latencies and syncer. If anyone's interested in this approach, I can provide the relevant code diffs. >2. Add INTR_FAST shims to the usb and ichsmb drivers so that neither >uses an ithread. The problem I ran into with this approach was that my interrupt handler needs to use psignal(9) - which requires PROC_LOCK() which (AFAIK) isn't allowed in an INTR_FAST handler. It would be useful if our interrupt subsystem allowed both INTR_FAST and normal interrupt handlers to be defined. If an INTR_FAST handler is defined then it gets executed and defines whether its associated interrupt thread handler needs to be triggered. If there's no INTR_FAST handler then the interrupt thread is always triggered. -- Peter Jeremy
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