Messages in this thread Patch in this message | | | Date | Tue, 19 Nov 2002 23:46:53 -0800 | From | Andrew Morton <> | Subject | the random driver |
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I'm seeing a context switch rate of 150/sec just writing a big file to disk at 20 megabytes/sec.
It is coming out of add_disk_randomness()'s invokation of batch_entropy_store().
That function is setting up deferred punt-to-process-context for every disk request, and has the potential to cause 1000 context switches per second. This is clearly excessive.
There is a 256 slot buffer in the random driver for this, and we are not using it at all effectively. I do intend to submit the below patch which will cause one context switch per 128 requests.
But this is a minimal fix. The batch_entropy_pool handling in there needs work.
a) It's racy. The head and tail pointers have no SMP protection and a race will cause it to dump 128 already-processed items back into the entropy pool.
b) It's weird. What's up with this?
batch_entropy_pool[2*batch_head] = a; batch_entropy_pool[(2*batch_head) + 1] = b;
It should be an array of 2-element structures.
c) The ring-buffer handling is awkward. It shouldn't be masking the head and tail pointers to always remain within bounds.
A better technique is to allow these indices to wrap at 0xffffffff and only mask their values when you actually use them as a subscript. This allows you to distinguish the completely-full case from the completely-empty one. See LOG_BUF* in kernel/printk.c.
d) It's punting work up to process context which could be performed right there in interrupt context.
My suggestion, if anyone cares, is to convert the entropy pool into smaller per-cpu buffers, protected by local_irq_save() only. This way the global lock (which isn't there yet) only needs to be taken when a CPU is actually dumping its buffer.
--- 25/drivers/char/random.c~reduce-random-context-switch-rate Tue Nov 19 23:17:12 2002 +++ 25-akpm/drivers/char/random.c Tue Nov 19 23:18:11 2002 @@ -663,7 +663,7 @@ void batch_entropy_store(u32 a, u32 b, i batch_entropy_credit[batch_head] = num; new = (batch_head+1) & (batch_max-1); - if (new != batch_tail) { + if ((new - batch_tail) >= batch_max / 2) { /* * Schedule it for the next timer tick: */ _ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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