On 06/25/2012 05:08 PM, Johannes Weiner wrote: > 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. Code page is very likely to share by other processes so I think it's very special than normal mmaped page. So I would like to raise bonus on code page than normal mmaped pages. So I would like to make following as if we can. Reclaim preference : unmapped page >> mapped page > VM_EXEC mapped page But at least, we can't solve Zheng's regression with current VM_EXEC protection logic because it seems he already used VM_EXEC tric :( I hope Erecalaimbe LRU list can solve 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>