On 02/27/2014 07:33 PM, Hugh Dickins wrote: > On Tue, 18 Feb 2014, Kelley Nielsen wrote: > >> The function try_to_unuse() is of quadratic complexity, with a lot of >> wasted effort. It unuses swap entries one by one, potentially iterating >> over all the page tables for all the processes in the system for each >> one. > > You've chosen a good target, and I like the look of what you've done. > But I'm afraid it will have to get uglier before it's ready, and I'm > unsure whether your approach will prove to be a clear win or not. I am more optimistic than you, because I have seen swapoff on my Nehalem system proceed at under 1MB/s for several hours, to clear maybe 3-4GB of stuff out of swap :) >> This new proposed implementation of try_to_unuse simplifies its >> complexity to linear. It iterates over the system's mms once, unusing >> all the affected entries as it walks each set of page tables. It also >> makes similar changes to shmem_unuse. >> >> Improvement >> >> swapoff was called on a swap partition containing about 50M of data, >> and calls to the function unuse_pte_range were counted. >> >> Present implementation....about 22.5M calls. >> Prototype.................about 7.0K calls. > > That's nice, but mostly it's the time spent that matters. > > I should explain why we've left the try_to_unuse() implementation as is > for so many years: it's a matter of tradeoff between fast cpu and slow > seeking disk. > I'll be surprised if your approach does not improve swapoff from SSD > (and brd and zram and zswap) very significantly; but the case to worry > about is swapoff from hard disk. You are changing swapoff to use the > cpu much more efficiently; but now that you no longer move linearly up > the swap_map, you are making the disk head seek around very much more. I suspect proper read-around of the swap area should take care of IO patterns well enough. The quadratic nature of the current try_to_unuse search can easily slow things down to comically low speeds... -- All rights reversed. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>