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? If we don't have any rationale, I would like to add new explicit API(ex, madvise(WORING_SET)) rather than depending VM internal implementation. > > Minchan suggested recently to have a separate LRU list for easily > reclaimable pages. If we balance the lists according to relative > size, we have pressure on mapped pages dictated by availability of > clean cache that is easier to reclaim. > > Rik, Minchan, what do you think? Yes. with Ereclaimable LRU list, we could do it. :) -- Kind regards, Minchan Kim -- 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>