On Mon, Jun 25, 2012 at 08:53:11AM +0900, Minchan Kim wrote: > Hi Hannes, > > On 06/23/2012 08:04 PM, Johannes Weiner wrote: > > > On Mon, May 21, 2012 at 07:07:00PM +0800, Zheng Liu wrote: > >> On Mon, May 21, 2012 at 11:37:05AM +0200, Johannes Weiner wrote: > >> [snip] > >>>>> Is it because the read()/write() IO is high throughput and pushes > >>>>> pages through the LRU lists faster than the mmap pages are referenced? > >>>> > >>>> Yes, in this application, one query needs to access mapped file page > >>>> twice and file page cache twice. Namely, one query needs to do 4 disk > >>>> I/Os. We have used fadvise(2) to reduce file page cache accessing to > >>>> only once. For mapped file page, in fact them are accessed only once > >>>> because in one query the same data is accessed twice. Thus, one query > >>>> causes 2 disk I/Os now. The size of read/write is quite larger than > >>>> mmap/munmap. So, as you see, if we can keep mmap/munmap file in memory > >>>> as much as possible, we will gain the better performance. > >>> > >>> You access the same unmapped cache twice, i.e. repeated reads or > >>> writes against the same file offset? > >> > >> No. We access the same mapped file twice. > >> > >>> > >>> How do you use fadvise? > >> > >> We access the header and content of the file respectively using read/write. > >> The header and content are sequentially. So we use fadivse(2) with > >> FADV_WILLNEED flag to do a readahead. > >> > >>>> In addition, another factor also has some impacts for this application. > >>>> In inactive_file_is_low_global(), it is different between 2.6.18 and > >>>> upstream kernel. IMHO, it causes that mapped file pages in active list > >>>> are moved into inactive list frequently. > >>>> > >>>> Currently, we add a parameter in inactive_file_is_low_global() to adjust > >>>> this ratio. Meanwhile we activate every mapped file pages for the first > >>>> time. Then the performance gets better, but it still doesn't reach the > >>>> performance of 2.6.18. > >>> > >>> 2.6.18 didn't have the active list protection at all and always > >>> forcibly deactivated pages during reclaim. Have you tried fully > >>> reverting to this by making inactive_file_is_low_global() return true > >>> unconditionally? > >> > >> No, I don't try it. AFAIK, 2.6.18 didn't protect the active list. But > >> it doesn't always forcibly deactivate the pages. I remember that in > >> 2.6.18 kernel we calculate 'mapped_ratio' in shrink_active_list(), and > >> then we get 'swap_tendency' according to 'mapped_ratio', 'distress', and > >> 'sc->swappiness'. If 'swap_tendency' is not greater than 100. It > >> doesn't reclaim mapped file pages. By this equation, if the sum of the > >> anonymous pages and mapped file pages is not greater than the 50% of > >> total pages, we don't deactivate these pages. Am I missing something? > > > > I think we need to go back to protecting mapped pages based on how > > much of reclaimable memory they make up, one way or another. > > > I partly agreed it with POV regression. > But I would like to understand rationale of "Why we should handle specially mmapped page". > In case of code pages(VM_EXEC), we already have handled it specially and > I understand why we did. At least, my opinion was that our LRU algorithm doesn't consider > _frequency_ fully while it does _recency_ well. I thought code page would be high frequency of access > compared to other pages. > But in case of mapped data pages, why we should handle specially? > I guess mapped data pages would have higher access chance than unmapped page because > unmapped page doesn't have any owner(it's just for caching for reducing I/O) while mapped page > has a owner above. > > Doesn't it make sense? I agree that the reason behind protecting VM_EXEC pages was that our frequency information for mapped pages is at LRU cycle granularity. But I don't see why you think this problem wouldn't apply to all mapped pages in general. -- 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>