On Sun, 16 September 2007 11:44:09 -0700, Linus Torvalds wrote: > On Sun, 16 Sep 2007, Jörn Engel wrote: > > > > My approach is to have one for mount points and ramfs/tmpfs/sysfs/etc. > > which are pinned for their entire lifetime and another for regular > > files/inodes. One could take a three-way approach and have > > always-pinned, often-pinned and rarely-pinned. > > > > We won't get never-pinned that way. > > That sounds pretty good. The problem, of course, is that most of the time, > the actual dentry allocation itself is done before you really know which > case the dentry will be in, and the natural place for actually giving the > dentry lifetime hint is *not* at "d_alloc()", but when we "instantiate" > it with d_add() or d_instantiate(). > > [...] > > And yes, you'd end up with the reallocation overhead quite often, but at > least it would now happen only when filling in a dentry, not in the > (*much* more critical) cached lookup path. There may be another approach. We could create a never-pinned cache, without trying hard to keep it full. Instead of moving a hot dentry at dput() time, we move a cold one from the end of lru. And if the lru list is short, we just chicken out. Our definition of "short lru list" can either be based on a ratio of pinned to unpinned dentries or on a metric of cache hits vs. cache misses. I tend to dislike the cache hit metric, because updatedb would cause tons of misses and result in the same mess we have right now. With this double cache, we have a source of slabs to cheaply reap under memory pressure, but still have a performance advantage (memcpy beats disk io by orders of magnitude). Jörn -- The story so far: In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. -- Douglas Adams - 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
