[sleepy.penguin] [PATCH 5/6] epoll: cache alignment for per-thread structure

[Eric Wong <normalperson@yhbt.net>]

This probably won't make a huge difference in Ruby, but perhaps
one day the unnecessary dirtying of cache lines will affect
performance (and we'll be ready when that day comes).

While we're at it, remove usage of pthread* functions for
thread-local variables.  The __thread construct from GCC (and
also implemented by clang) is much easier-to-use than the
pthread_*specific API.
---
 ext/sleepy_penguin/epoll.c | 46 +++++++++++++++++++++-------------------------
 1 file changed, 21 insertions(+), 25 deletions(-)

diff --git a/ext/sleepy_penguin/epoll.c b/ext/sleepy_penguin/epoll.c
index 64df698..70a77f6 100644
--- a/ext/sleepy_penguin/epoll.c
+++ b/ext/sleepy_penguin/epoll.c
@@ -1,14 +1,15 @@
 #include "sleepy_penguin.h"
 #include <sys/epoll.h>
-#include <pthread.h>
+#include <unistd.h>
 #include <time.h>
 #include "missing_epoll.h"
 #include "missing_rb_thread_fd_close.h"
 #include "missing_rb_update_max_fd.h"
+#define L1_CACHE_LINE_MAX 128 /* largest I've seen (Pentium 4) */
 
-static pthread_key_t epoll_key;
 static ID id_for_fd;
 static VALUE cEpoll;
+static size_t l1_cache_line_size;
 
 static uint64_t now_ms(void)
 {
@@ -56,9 +57,10 @@ static void ep_fd_check(struct ep_per_thread *ept)
 
 static struct ep_per_thread *ept_get(VALUE self, int maxevents)
 {
-	struct ep_per_thread *ept = pthread_getspecific(epoll_key);
+	static __thread struct ep_per_thread *ept;
 	size_t size;
 	int err;
+	void *ptr;
 
 	if (ept && ept->capa >= maxevents)
 		goto out;
@@ -67,14 +69,12 @@ static struct ep_per_thread *ept_get(VALUE self, int maxevents)
 	       sizeof(struct epoll_event) * maxevents;
 
 	free(ept); /* free(NULL) is POSIX and works on glibc */
-	ept = malloc(size);
-	if (ept == NULL)
-		rb_memerror();
-	err = pthread_setspecific(epoll_key, ept);
-	if (err != 0) {
+	err = posix_memalign(&ptr, l1_cache_line_size, size);
+	if (err) {
 		errno = err;
-		rb_sys_fail("pthread_setspecific");
+		rb_memerror();
 	}
+	ept = ptr;
 	ept->capa = maxevents;
 out:
 	ept->maxevents = maxevents;
@@ -230,32 +230,28 @@ static VALUE epwait(int argc, VALUE *argv, VALUE self)
 	return real_epwait(ept);
 }
 
-static void epoll_once(void)
-{
-	int err = pthread_key_create(&epoll_key, free);
-
-	if (err) {
-		errno = err;
-		rb_sys_fail("pthread_key_create");
-	}
-}
-
 /* :nodoc: */
 static VALUE event_flags(VALUE self, VALUE flags)
 {
 	return UINT2NUM(rb_sp_get_uflags(self, flags));
 }
 
+static size_t l1_cache_line_size_detect(void)
+{
+#ifdef _SC_LEVEL1_DCACHE_LINESIZE
+	long tmp = sysconf(_SC_LEVEL1_DCACHE_LINESIZE);
+
+	if (tmp > 0 && tmp <= L1_CACHE_LINE_MAX)
+		return (size_t)tmp;
+#endif /* _SC_LEVEL1_DCACHE_LINESIZE */
+	return L1_CACHE_LINE_MAX;
+}
+
 void sleepy_penguin_init_epoll(void)
 {
 	VALUE mSleepyPenguin, cEpoll_IO;
-	static pthread_once_t once = PTHREAD_ONCE_INIT;
-	int err = pthread_once(&once, epoll_once);
 
-	if (err) {
-		errno = err;
-		rb_sys_fail("pthread_once(.., epoll_once)");
-	}
+	l1_cache_line_size = l1_cache_line_size_detect();
 
 	/*
 	 * Document-module: SleepyPenguin
-- 
1.8.2.279.g631bc94