On 04/01/2011 09:17 AM, KOSAKI Motohiro wrote: > Hi Christoph, > > Thanks, long explanation. > > >> On Thu, 31 Mar 2011, KOSAKI Motohiro wrote: >> >>> 1) zone reclaim doesn't work if the system has multiple node and the >>> workload is file cache oriented (eg file server, web server, mail server, et al). >>> because zone recliam make some much free pages than zone->pages_min and >>> then new page cache request consume nearest node memory and then it >>> bring next zone reclaim. Then, memory utilization is reduced and >>> unnecessary LRU discard is increased dramatically. >> >> That is only true if the webserver only allocates from a single node. If >> the allocation load is balanced then it will be fine. It is useful to >> reclaim pages from the node where we allocate memory since that keeps the >> dataset node local. > > Why? > Scheduler load balancing only consider cpu load. Then, usually memory > pressure is no complete symmetric. That's the reason why we got the > bug report periodically. Agreed. As Christoph said if the allocation load is balanced it will be fine. But I think it's not always true that the allocation load is balanced. >>> But, I agree that now we have to concern slightly large VM change parhaps >>> (or parhaps not). Ok, it's good opportunity to fill out some thing. >>> Historically, Linux MM has "free memory are waste memory" policy, and It >>> worked completely fine. But now we have a few exceptions. >>> >>> 1) RT, embedded and finance systems. They really hope to avoid reclaim >>> latency (ie avoid foreground reclaim completely) and they can accept >>> to make slightly much free pages before memory shortage. >> >> In general we need a mechanism to ensure we can avoid reclaim during >> critical sections of application. So some way to give some hints to the >> machine to free up lots of memory (/proc/sys/vm/dropcaches is far too >> drastic) may be useful. > > Exactly. > I've heard multiple times this request from finance people. And I've also > heared the same request from bullet train control software people recently. I completely agree with you. I have both customers and they really need it to make their critical section deterministic. Thanks, Satoru -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
