So I've been thinking about this some more, and it seems to me is actually, what we need is *both* an LRU and a RR scheme. The real problem here is that we have workloads that are generating a large number of "low value" extent cache entries. That is, they are extremely unlikely to be used again, because they are small, and being generated when you have a highly fragmented extent status cache, and very often, the workload is a random read/write workload, so there is no way the full "working set" of extent cache entries could be kept in memory at the same time anyway. These less valuable cache entries are being generated at a very high rate, and we want to make sure we don't penalize the "valuable" cache entries. There's a classic solution to this problem for garbage collectors, and that's to have a "nursery" and "tenured" space. So what we could do is to have two lists (as the proposed LRU improvement patch does), but in the first list, we put the delalloc and "tenured" cache entries, and in the second list we put the "nursery" cache entries. The "nursery" cache items are cleaned using an RR scheme, and indeed, we might want to have a system where we try to keep the "nursery" cache items to a mangeable level, even if we aren't under memory pressure. If a cache item gets used a certain number of times, then when we get to that item in the RR scheme, it gets "promoted" to the "tenured" space. The "tenured" space is then kept under control using some kind of LRU scheme, and a target number of "tenured" items. (We might or might not want to count delalloc entries for the purposes of this target. That's TBD.) The system should ideally automatically tune itself to control the promotion rate from the nursery to tenured space based on the number of uses required before a cache entry gets promoted, and there will be a bunch of hueristics that we'll need to tune. But I think this general approach should work pretty well. What do other people think? - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
