On Tue, 19 Jul 2011 15:54:23 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote: > We have already acknowledged that swapoff of a tmpfs file is slower > than it was before conversion to the generic radix_tree: a little > slower there will be acceptable, if the hotter paths are faster. > > But it was a shock to find swapoff of a 500MB file 20 times slower > on my laptop, taking 10 minutes; and at that rate it significantly > slows down my testing. So it used to take half a minute? That was already awful. Why? Was it IO-bound? It doesn't sound like it. > Now, most of that turned out to be overhead from PROVE_LOCKING and > PROVE_RCU: without those it was only 4 times slower than before; > and more realistic tests on other machines don't fare as badly. What's unrealistic about doing swapoff of a 500MB tmpfs file? Also, confused. You're talking about creating a regular file on tmpfs and then using that as a swapfile? If so, that's a kernel-hacker-curiosity only? > I've tried a number of things to improve it, including tagging the > swap entries, then doing lookup by tag: I'd expected that to halve > the time, but in practice it's erratic, and often counter-productive. > > The only change I've so far found to make a consistent improvement, > is to short-circuit the way we go back and forth, gang lookup packing > entries into the array supplied, then shmem scanning that array for the > target entry. Scanning in place doubles the speed, so it's now only > twice as slow as before (or three times slower when the PROVEs are on). > > So, add radix_tree_locate_item() as an expedient, once-off, single-caller > hack to do the lookup directly in place. #ifdef it on CONFIG_SHMEM and > CONFIG_SWAP, as much to document its limited applicability as save space > in other configurations. And, sadly, #include sched.h for cond_resched(). > How much did that 10 minutes improve? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>