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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] Re: [PATCH V4 2/3] qemu: Generic task offloading framework: threadlets |
Date: | Wed, 16 Jun 2010 10:20:36 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100423 Lightning/1.0b1 Thunderbird/3.0.4 |
On 06/16/2010 09:52 AM, Paolo Bonzini wrote:
On 06/16/2010 04:38 PM, Anthony Liguori wrote:On 06/16/2010 09:29 AM, Paolo Bonzini wrote:On 06/16/2010 04:22 PM, Jamie Lokier wrote:Paolo Bonzini wrote:These should be (at least for now) block-obj-$(CONFIG_POSIX).+ while (QTAILQ_EMPTY(&(queue->request_list))&& + (ret != ETIMEDOUT)) { + ret = qemu_cond_timedwait(&(queue->cond), + &(queue->lock), 10*100000); + }Using qemu_cond_timedwait is a hack for not properly broadcasting the condvar in flush_threadlet_queue.Are you sure? It looks like it also expires idle threads after a fixed amount of idle time.Unnecessary idle threads are immediately expired as soon as the threadlet exits if ncecessary, since hereIf a threadlet is waiting to consume more work, unless we do a pthread_cancel (I dislike cancellation) it will keep waiting until it gets more work (which would mean it's not actually idle)...Agreed---no cancellation, please.BTW it's obviously okay with signaling the condition when a threadlet is submitted. But when something affects all queue's workers (flush_threadlet_queue) you want a broadcast and using expiration as a substitute is fishy.
IMHO, there shouldn't be a need for flush_threadlet_queue. It doesn't look used in the aio conversion and if virtio-9p needs it, I suspect something is wrong.
Regards, Anthony Liguori
+ queue->idle_threads++; + +check_exit: + if (queue->exit || ((queue->idle_threads > 0) && + (queue->cur_threads > queue->min_threads))) { + /* We exit the queue or we retain minimum number of threads */ + break; + } queue->idle_threads > 0 will always be true (so maybe that should be changed into an assertion: "this thread is idle, so there must be idle threads").queue->exit could be true though so it's necessary to at least check that condition.Yes, of course. The correct test should be: if (queue->exit || queue->cur_threads > queue->min_threads)But queue->idle_threads will be > 0 even if coming via the goto (which should be eliminated).Or maybe no. After flushing you still want min_threads threads to run. The correct thing then would be:do { ... assert (queue->idle_threads > 0); if (queue->exit) { /* Threads waiting on the barrier cannot do work. */ queue->idle_threads--; qemu_mutex_unlock(&(queue->lock)); qemu_barrier_wait(&queue->barr); qemu_mutex_lock(&(queue->lock)); queue->idle_threads++; } } while (queue->cur_threads <= queue->min_threads); queue->idle_threads--; queue->cur_threads--; qemu_mutex_unlock(&queue->lock); return NULL;So, if min_threads were changed, broadcasting the condition would be enough to exit unwanted threads one at a time, as soon as it grabs the lock.Paolo
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