On Tue, 2 Feb 2010, david@xxxxxxx wrote: > On Tue, 2 Feb 2010, Linus Torvalds wrote: > > > > Also, keep in mind that read-ahead is not always a win. It can be a huge > > loss too. Which is why we have _heuristics_. They fundamentally cannot > > catch every case, but what they aim for is to do a good job on average. > > as a note from the field, I just had an application that needed to be changed > because it did excessive read-ahead. it turned a 2 min reporting run into a 20 > min reporting run because for this report the access was really random and the > app forced large read-ahead. Yeah. And the reason Wu did this patch is similar: something that _should_ have taken just quarter of a second took about 7 seconds, because read-ahead triggered on this really slow device that only feeds about 15kB/s (yes, _kilo_byte, not megabyte). You can always use POSIX_FADVISE_RANDOM to disable it, but it's seldom something that people do. And there are real loads that have random components to them without being _entirely_ random, so in an optimal world we should just have heuristics that work well. The problem is, it's often easier to test/debug the "good" cases, ie the cases where we _want_ read-ahead to trigger. So that probably means that we have a tendency to read-ahead too aggressively, because those cases are the ones where people can most easily look at it and say "yeah, this improves throughput of a 'dd bs=8192'". So then when we find loads where read-ahead hurts, I think we need to take _that_ case very seriously. Because otherwise our selection bias for testing read-ahead will fail. Linus -- 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
