Messages in this thread Patch in this message | | | Date | Tue, 2 Jan 2001 15:59:26 +0100 (CET) | From | Mike Galbraith <> | Subject | Re: scheduling problem? |
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On Wed, 3 Jan 2001, Anton Blanchard wrote:
> > Hi Mike, > > > I am seeing (what I believe is;) severe process CPU starvation in > > 2.4.0-prerelease. At first, I attributed it to semaphore troubles > > as when I enable semaphore deadlock detection in IKD and set it to > > 5 seconds, it triggers 100% of the time on nscd when I do sequential > > I/O (iozone eg). In the meantime, I've done a slew of tracing, and > > I think the holder of the semaphore I'm timing out on just flat isn't > > being scheduled so it can release it. In the usual case of nscd, I > > _think_ it's another nscd holding the semaphore. In no trace can I > > go back far enough to catch the taker of the semaphore or any user > > task other than iozone running between __down() time and timeout 5 > > seconds later. (trace buffer covers ~8 seconds of kernel time) > > Did this just appear in recent kernels? Maybe bdflush was hiding the > situation in earlier kernels as it would cause io hogs to block when > things got only mildly interesting.
Yes and no. I've seen nasty stalls for quite a while now. (I think that there is a wakeup problem lurking)
I found the change which triggers my horrid stalls. Nobody is going to believe this...
diff -urN linux-2.4.0-test13-pre6/fs/buffer.c linux-2.4.0-test13-pre7/fs/buffer.c --- linux-2.4.0-test13-pre6/fs/buffer.c Sat Dec 30 08:58:56 2000 +++ linux-2.4.0-test13-pre7/fs/buffer.c Sun Dec 31 06:22:31 2000 @@ -122,16 +122,17 @@ when trying to refill buffers. */ int interval; /* jiffies delay between kupdate flushes */ int age_buffer; /* Time for normal buffer to age before we flush it */ - int dummy1; /* unused, was age_super */ + int nfract_sync; /* Percentage of buffer cache dirty to + activate bdflush synchronously */ int dummy2; /* unused */ int dummy3; /* unused */ } b_un; unsigned int data[N_PARAM]; -} bdf_prm = {{40, 500, 64, 256, 5*HZ, 30*HZ, 5*HZ, 1884, 2}}; +} bdf_prm = {{40, 500, 64, 256, 5*HZ, 30*HZ, 80, 0, 0}}; /* These are the min and max parameter values that we will allow to be assigned */ -int bdflush_min[N_PARAM] = { 0, 10, 5, 25, 0, 1*HZ, 1*HZ, 1, 1}; -int bdflush_max[N_PARAM] = {100,50000, 20000, 20000,600*HZ, 6000*HZ, 6000*HZ, 2047, 5}; +int bdflush_min[N_PARAM] = { 0, 10, 5, 25, 0, 1*HZ, 0, 0, 0}; +int bdflush_max[N_PARAM] = {100,50000, 20000, 20000,600*HZ, 6000*HZ, 100, 0, 0}; /* * Rewrote the wait-routines to use the "new" wait-queue functionality, @@ -1032,9 +1034,9 @@ dirty = size_buffers_type[BUF_DIRTY] >> PAGE_SHIFT; tot = nr_free_buffer_pages(); - dirty *= 200; + dirty *= 100; soft_dirty_limit = tot * bdf_prm.b_un.nfract; - hard_dirty_limit = soft_dirty_limit * 2; + hard_dirty_limit = tot * bdf_prm.b_un.nfract_sync; /* First, check for the "real" dirty limit. */ if (dirty > soft_dirty_limit) { ...but reversing this cures my semaphore timeouts. Don't say impossible :) I didn't believe it either until I retested several times. I wager that if I just fiddle with parameters I'll be able to make the problem come and go at will. (means the real problem is gonna be a weird one:)
-Mike
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