I went to merge this but then had a question on part of it (below).
On Wed, Feb 1, 2012 at 7:54 AM, Jim Schutt <[email protected]> wrote:
> Under heavy write load from many clients, many reader threads will
> be waiting in the policy throttler, all on a single condition variable.
> When a wakeup is signalled, any of those threads may receive the
> signal. This increases the variance in the message processing
> latency, and in extreme cases can significantly delay a message.
>
> This patch causes threads to exit a throttler in the same order
> they entered.
>
> Signed-off-by: Jim Schutt <[email protected]>
> ---
> src/common/Throttle.h | 42 ++++++++++++++++++++++++++++--------------
> 1 files changed, 28 insertions(+), 14 deletions(-)
>
> diff --git a/src/common/Throttle.h b/src/common/Throttle.h
> index 10560bf..ca72060 100644
> --- a/src/common/Throttle.h
> +++ b/src/common/Throttle.h
> @@ -3,23 +3,31 @@
>
> #include "Mutex.h"
> #include "Cond.h"
> +#include <list>
>
> class Throttle {
> - int64_t count, max, waiting;
> + int64_t count, max;
> uint64_t sseq, wseq;
> Mutex lock;
> - Cond cond;
> + list<Cond*> cond;
>
> public:
> - Throttle(int64_t m = 0) : count(0), max(m), waiting(0), sseq(0), wseq(0),
> + Throttle(int64_t m = 0) : count(0), max(m), sseq(0), wseq(0),
> lock("Throttle::lock") {
> assert(m >= 0);
> }
> + ~Throttle() {
> + while (!cond.empty()) {
> + Cond *cv = cond.front();
> + delete cv;
> + cond.pop_front();
> + }
> + }
>
> private:
> void _reset_max(int64_t m) {
> - if (m < max)
> - cond.SignalOne();
> + if (m < max && !cond.empty())
> + cond.front()->SignalOne();
> max = m;
> }
> bool _should_wait(int64_t c) {
> @@ -28,19 +36,24 @@ private:
> ((c < max && count + c > max) || // normally stay under max
> (c >= max && count > max)); // except for large c
> }
> +
> bool _wait(int64_t c) {
> bool waited = false;
> - if (_should_wait(c)) {
> - waiting += c;
> + if (_should_wait(c) || !cond.empty()) { // always wait behind other
> waiters.
> + Cond *cv = new Cond;
> + cond.push_back(cv);
> do {
> + if (cv != cond.front())
> + cond.front()->SignalOne(); // wake up the oldest.
What's this extra wakeup for? Unless I'm missing something it's always
going to be gratuitous. :/
> waited = true;
> - cond.Wait(lock);
> - } while (_should_wait(c));
> - waiting -= c;
> + cv->Wait(lock);
> + } while (_should_wait(c) || cv != cond.front());
> + delete cv;
> + cond.pop_front();
>
> // wake up the next guy
> - if (waiting)
> - cond.SignalOne();
> + if (!cond.empty())
> + cond.front()->SignalOne();
> }
> return waited;
> }
> @@ -101,7 +114,7 @@ public:
> bool get_or_fail(int64_t c = 1) {
> assert (c >= 0);
> Mutex::Locker l(lock);
> - if (_should_wait(c)) return false;
> + if (_should_wait(c) || !cond.empty()) return false;
> count += c;
> return true;
> }
> @@ -110,7 +123,8 @@ public:
> assert(c >= 0);
> Mutex::Locker l(lock);
> if (c) {
> - cond.SignalOne();
> + if (!cond.empty())
> + cond.front()->SignalOne();
> count -= c;
> assert(count >= 0); //if count goes negative, we failed somewhere!
> }
> --
> 1.7.1
>
>
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