On Mon, Jun 06, 2011 at 01:46:00PM -0400, Mathieu Desnoyers wrote:
> If I remove the URCU_CALL_RCU_RT flag from the rbtree single writer
> test, thus using the pthread_cond_signal mechanism, there is a huge
> slowdown: without cpu affinity for the worker threads, it crawls to 129
> updates/s (looks like mutex contention between the thread calling
> call_rcu and the call_rcu thread). Adding CPU affinity to the per-cpu
> call_rcu threads, I get 546 updates/s, which is slightly better (better
> cache locality, and maybe the mutex contention is not as bad thanks to
> the two threads sharing the same CPU).
>
> So I decided to try replacing pthread_cond_wait/signal with my
> futex-based implementation I use for the rest of the urcu lib: it has
> the advantage of removing the mutex from the call_rcu() execution
> entirely, sampling the "futex" variable without any mutex whatsoever for
> the case where no wakeup is needed.
>
> Disabling URCU_CALL_RCU_RT flag, with per-cpu affined call_rcu threads,
> with my futex-based wakeup implementation, I get 55754 updates/s (even
> better than with URCU_CALL_RCU_RT flag!).
Assuming that compat_futex.c handles the non-Linux case, works for me!
Thanx, Paul
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> ---
> urcu-call-rcu-impl.h | 56
> +++++++++++++++++++++++++--------------------------
> 1 file changed, 28 insertions(+), 28 deletions(-)
>
> Index: userspace-rcu/urcu-call-rcu-impl.h
> ===================================================================
> --- userspace-rcu.orig/urcu-call-rcu-impl.h
> +++ userspace-rcu/urcu-call-rcu-impl.h
> @@ -39,6 +39,7 @@
> #include "urcu-call-rcu.h"
> #include "urcu-pointer.h"
> #include "urcu/list.h"
> +#include "urcu/urcu-futex.h"
>
> /* Data structure that identifies a call_rcu thread. */
>
> @@ -46,7 +47,7 @@ struct call_rcu_data {
> struct cds_wfq_queue cbs;
> unsigned long flags;
> pthread_mutex_t mtx;
> - pthread_cond_t cond;
> + int futex;
> unsigned long qlen;
> pthread_t tid;
> int cpu_affinity;
> @@ -88,6 +89,26 @@ static struct call_rcu_data *default_cal
> static struct call_rcu_data **per_cpu_call_rcu_data;
> static long maxcpus;
>
> +static void call_rcu_wait(struct call_rcu_data *crdp)
> +{
> + /* Read call_rcu list before read futex */
> + cmm_smp_mb();
> + if (uatomic_read(&crdp->futex) == -1)
> + futex_async(&crdp->futex, FUTEX_WAIT, -1,
> + NULL, NULL, 0);
> +}
> +
> +static void call_rcu_wake_up(struct call_rcu_data *crdp)
> +{
> + /* Write to call_rcu list before reading/writing futex */
> + cmm_smp_mb();
> + if (unlikely(uatomic_read(&crdp->futex) == -1)) {
> + uatomic_set(&crdp->futex, 0);
> + futex_async(&crdp->futex, FUTEX_WAKE, 1,
> + NULL, NULL, 0);
> + }
> +}
> +
> /* Allocate the array if it has not already been allocated. */
>
> static void alloc_cpu_call_rcu_data(void)
> @@ -219,19 +240,9 @@ static void *call_rcu_thread(void *arg)
> if (crdp->flags & URCU_CALL_RCU_RT)
> poll(NULL, 0, 10);
> else {
> - call_rcu_lock(&crdp->mtx);
> - _CMM_STORE_SHARED(crdp->flags,
> - crdp->flags & ~URCU_CALL_RCU_RUNNING);
> - if (&crdp->cbs.head ==
> - _CMM_LOAD_SHARED(crdp->cbs.tail) &&
> - pthread_cond_wait(&crdp->cond, &crdp->mtx) != 0) {
> - perror("pthread_cond_wait");
> - exit(-1);
> - }
> - _CMM_STORE_SHARED(crdp->flags,
> - crdp->flags | URCU_CALL_RCU_RUNNING);
> + if (&crdp->cbs.head == _CMM_LOAD_SHARED(crdp->cbs.tail))
> + call_rcu_wait(crdp);
> poll(NULL, 0, 10);
> - call_rcu_unlock(&crdp->mtx);
> }
> }
> call_rcu_lock(&crdp->mtx);
> @@ -264,11 +275,8 @@ static void call_rcu_data_init(struct ca
> perror("pthread_mutex_init");
> exit(-1);
> }
> - if (pthread_cond_init(&crdp->cond, NULL) != 0) {
> - perror("pthread_cond_init");
> - exit(-1);
> - }
> - crdp->flags = flags | URCU_CALL_RCU_RUNNING;
> + crdp->futex = 0;
> + crdp->flags = flags;
> cds_list_add(&crdp->list, &call_rcu_data_list);
> crdp->cpu_affinity = cpu_affinity;
> cmm_smp_mb(); /* Structure initialized before pointer is planted. */
> @@ -492,16 +500,8 @@ int create_all_cpu_call_rcu_data(unsigne
> */
> static void wake_call_rcu_thread(struct call_rcu_data *crdp)
> {
> - if (!(_CMM_LOAD_SHARED(crdp->flags) & URCU_CALL_RCU_RT)) {
> - call_rcu_lock(&crdp->mtx);
> - if (!(_CMM_LOAD_SHARED(crdp->flags) & URCU_CALL_RCU_RUNNING)) {
> - if (pthread_cond_signal(&crdp->cond) != 0) {
> - perror("pthread_cond_signal");
> - exit(-1);
> - }
> - }
> - call_rcu_unlock(&crdp->mtx);
> - }
> + if (!(_CMM_LOAD_SHARED(crdp->flags) & URCU_CALL_RCU_RT))
> + call_rcu_wake_up(crdp);
> }
>
> /*
>
>
> _______________________________________________
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> http://svcs.cs.pdx.edu/mailman/listinfo/rp
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