On Mon, Sep 13, 2010 at 06:48:10PM +0900, Minchan Kim wrote: > >> > > > <SNIP> > >> > > > I'm not saying it is. The objective is to identify a situation where > >> > > > sleeping until the next write or congestion clears is pointless. We have > >> > > > already identified that we are not congested so the question is "are we > >> > > > writing a lot at the moment?". The assumption is that if there is a lot > >> > > > of writing going on, we might as well sleep until one completes rather > >> > > > than reclaiming more. > >> > > > > >> > > > This is the first effort at identifying pointless sleeps. Better ones > >> > > > might be identified in the future but that shouldn't stop us making a > >> > > > semi-sensible decision now. > >> > > > >> > > nr_bdi_congested is no problem since we have used it for a long time. > >> > > But you added new rule about writeback. > >> > > > >> > > >> > Yes, I'm trying to add a new rule about throttling in the page allocator > >> > and from vmscan. As you can see from the results in the leader, we are > >> > currently sleeping more than we need to. > >> > >> I can see the about avoiding congestion_wait but can't find about > >> (writeback < incative / 2) hueristic result. > >> > > > > See the leader and each of the report sections entitled > > "FTrace Reclaim Statistics: congestion_wait". It provides a measure of > > how sleep times are affected. > > > > "congest waited" are waits due to calling congestion_wait. "conditional waited" > > are those related to wait_iff_congested(). As you will see from the reports, > > sleep times are reduced overall while callers of wait_iff_congested() still > > go to sleep. The reports entitled "FTrace Reclaim Statistics: vmscan" show > > how reclaim is behaving and indicators so far are that reclaim is not hurt > > by introducing wait_iff_congested(). > > I saw the result. > It was a result about effectiveness _both_ nr_bdi_congested and > (writeback < inactive/2). > What I mean is just effectiveness (writeback < inactive/2) _alone_. I didn't measured it because such a change means that wait_iff_congested() ignored BDI congestion. If we were reclaiming on a NUMA machine for example, it could mean that a BDI gets flooded with requests if we only checked the ratios of one zone if little writeback was happening in that zone at the time. It did not seem like a good idea to ignore congestion. > If we remove (writeback < inactive / 2) check and unconditionally > return, how does the behavior changed? > Based on just the workload Johannes sent, scanning and completion times both increased without any improvement in the scanning/reclaim ratio (a bad result) hence why this logic was introduced to back off where there is some writeback taking place even if the BDI is not congested. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
