On Tue, Jan 08, 2013 at 05:38:53PM +0900, Minchan Kim wrote: > On Tue, Jan 08, 2013 at 03:32:29PM +0800, Shaohua Li wrote: > > On Tue, Jan 08, 2013 at 02:38:56PM +0900, Minchan Kim wrote: > > > Hi Shaohua, > > > > > > On Tue, Jan 08, 2013 at 12:26:09PM +0800, Shaohua Li wrote: > > > > On Tue, Jan 08, 2013 at 10:16:07AM +0800, Wanpeng Li wrote: > > > > > On Mon, Jan 07, 2013 at 12:06:30PM -0800, Andrew Morton wrote: > > > > > >On Mon, 7 Jan 2013 16:12:37 +0800 > > > > > >Shaohua Li <shli@xxxxxxxxxx> wrote: > > > > > > > > > > > >> > > > > > >> Make madvise(MADV_WILLNEED) support swap file prefetch. If memory is swapout, > > > > > >> this syscall can do swapin prefetch. It has no impact if the memory isn't > > > > > >> swapout. > > > > > > > > > > > >Seems sensible. > > > > > > > > > > Hi Andrew and Shaohua, > > > > > > > > > > What's the performance in the scenario of serious memory pressure? Since > > > > > in this case pages in swap are highly fragmented and cache hit is most > > > > > impossible. If WILLNEED path should add a check to skip readahead in > > > > > this case since swapin only leads to unnecessary memory allocation. > > > > > > > > pages in swap are not highly fragmented if you access memory sequentially. In > > > > that case, the pages you accessed will be added to lru list side by side. So if > > > > app does swap prefetch, we can do sequential disk access and merge small > > > > request to big one. > > > > > > How can you make sure that the range of WILLNEED was always sequentially accesssed? > > > > you can't guarantee this even for file access. > > Indeed. > > > > > > > Another advantage is prefetch can drive high disk iodepth. For sequential > > > > > > What does it mean 'iodepth'? I failed to grep it in google. :( > > > > io depth. How many requests are inflight at a givin time. > > Thanks for the info! > > > > > > > access, this can cause big request. Even for random access, high iodepth has > > > > much better performance especially for SSD. > > > > > > So you mean WILLNEED is always good in where both random and sequential in "SSD"? > > > Then, how about the "Disk"? > > > > Hmm, even for hard disk, high iodepth random access is faster than single > > iodepth access. Today's disk is NCQ disk. But the speedup isn't that > > significant like a SSD. For sequential access, both harddisk and SSD have > > better performance with higher iodepth. > > > > > Wanpeng's comment makes sense to me so I guess others can have a same question > > > about this patch. So it would be better to write your rationale in changelog. > > > > I would, but the question is just like why app wants to prefetch file pages. I > > thought it's commonsense. The problem like memory allocation exists in file > > prefetch too. The advantages (better IO access, CPU and disk can operate in > > parallel and so on) apply for both file and swap prefetch. > > Agreed. But I have a question about semantic of madvise(DONTNEED) of anon vma. > If Linux start to support it for anon, user can misunderstand it following as. > > User might think we start to use anonymous pages in that range soon so he > gives the hint to kernel to map all pages of the range to page table in advance. > (ie, pre page fault like MAP_POPULATE) and if one of the page might be > swapped out, readahead it. What do you think about it? > For clarification, it would be better to add man page description with Ccing > man page maintainer. there is no confusion if the page exists or swapped. I thought what you are are thinking about is the page isn't populated yet. The manpage declaims WILLNEED "it might be a good idea to read some pages ahead." This sounds clear this isn't to populate memory and matches what we did. But I'm not sure what's the precise description. Thanks, Shaohua -- 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>