On Thu, Nov 30, 2017 at 08:43:41AM -0500, Waiman Long wrote: > On 11/29/2017 07:53 PM, Minchan Kim wrote: > > Hello, > > > > On Wed, Nov 29, 2017 at 09:17:34AM -0500, Waiman Long wrote: > >> The list_lru_del() function removes the given item from the LRU list. > >> The operation looks simple, but it involves writing into the cachelines > >> of the two neighboring list entries in order to get the deletion done. > >> That can take a while if the cachelines aren't there yet, thus > >> prolonging the lock hold time. > >> > >> To reduce the lock hold time, the cachelines of the two neighboring > >> list entries are now prefetched before acquiring the list_lru_node's > >> lock. > >> > >> Using a multi-threaded test program that created a large number > >> of dentries and then killed them, the execution time was reduced > >> from 38.5s to 36.6s after applying the patch on a 2-socket 36-core > >> 72-thread x86-64 system. > >> > >> Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > >> --- > >> mm/list_lru.c | 10 +++++++++- > >> 1 file changed, 9 insertions(+), 1 deletion(-) > >> > >> diff --git a/mm/list_lru.c b/mm/list_lru.c > >> index f141f0c..65aae44 100644 > >> --- a/mm/list_lru.c > >> +++ b/mm/list_lru.c > >> @@ -132,8 +132,16 @@ bool list_lru_del(struct list_lru *lru, struct list_head *item) > >> struct list_lru_node *nlru = &lru->node[nid]; > >> struct list_lru_one *l; > >> > >> + /* > >> + * Prefetch the neighboring list entries to reduce lock hold time. > >> + */ > >> + if (unlikely(list_empty(item))) > >> + return false; > >> + prefetchw(item->prev); > >> + prefetchw(item->next); > >> + > > A question: > > > > A few month ago, I had a chance to measure prefetch effect with my testing > > workload. For the clarification, it's not list_lru_del but list traverse > > stuff so it might be similar. > > > > With my experiment at that time, it was really hard to find best place to > > add prefetchw. Sometimes, it was too eariler or late so the effect was > > not good, even worse on some cases. > > > > Also, the performance was different with each machine although my testing > > machines was just two. ;-) > > > > So my question is what's a rule of thumb to add prefetch command? > > Like your code, putting prefetch right before touching? > > > > I'm really wonder what's the rule to make every arch/machines happy > > with prefetch. > > I add the prefetchw() before spin_lock() because the latency of the > lockinig operation can be highly variable. There will have high latency > when the lock is contended. With the prefetch, lock hold time will be > reduced. In turn, it helps to reduce the amount of lock contention as > well. If there is no lock contention, the prefetch won't help. I knew it by your description. My point is prefetch optimization could show different results by various architectures and workloads so I wanted to know what kinds of rule we have to prove it's always win or no harmful for *everycase* in geneal. This is a performance patch and it's very micro-optimized topic so I think we need more data to prove it. Maybe perf is best friend and need a experiment with no lock contention case, at least. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
