On Fri, 19 Feb 2016, Konstantin Khlebnikov wrote: > On Fri, Feb 19, 2016 at 1:57 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > On Thu, 18 Feb 2016, Johannes Weiner wrote: > > > >> Even before we added MemAvailable, users knew that page cache is > >> easily convertible to free memory on pressure, and estimated their > >> "available" memory by looking at the sum of MemFree, Cached, Buffers. > >> However, "Cached" is calculated using NR_FILE_PAGES, which includes > >> shmem and random driver pages inserted into the page tables; neither > >> of which are easily reclaimable, or reclaimable at all. Reclaiming > >> shmem requires swapping, which is slow. And unlike page cache, which > >> has fairly conservative dirty limits, all of shmem needs to be written > >> out before becoming evictable. Without swap, shmem is not evictable at > >> all. And driver pages certainly never are. > >> > >> Calling these pages "Cached" is misleading and has resulted in broken > >> formulas in userspace. They misrepresent the memory situation and > >> cause either waste or unexpected OOM kills. With 64-bit and per-cpu > >> memory we are way past the point where the relationship between > >> virtual and physical memory is meaningful and users can rely on > >> overcommit protection. OOM kills can not be avoided without wasting > >> enormous amounts of memory this way. This shifts the management burden > >> toward userspace, toward applications monitoring their environment and > >> adjusting their operations. And so where statistics like /proc/meminfo > >> used to be more informational, we have more and more software relying > >> on them to make automated decisions based on utilization. > >> > >> But if userspace is supposed to take over responsibility, it needs a > >> clear and accurate kernel interface to base its judgement on. And one > >> of the requirements is certainly that memory consumers with wildly > >> different reclaimability are not conflated. Adding MemAvailable is a > >> good step in that direction, but there is software like Sigar[1] in > >> circulation that might not get updated anytime soon. And even then, > >> new users will continue to go for the intuitive interpretation of the > >> Cached item. We can't blame them. There are years of tradition behind > >> it, starting with the way free(1) and vmstat(8) have always reported > >> free, buffers, cached. And try as we might, using "Cached" for > >> unevictable memory is never going to be obvious. > >> > >> The semantics of Cached including shmem and kernel pages have been > >> this way forever, dictated by the single-LRU implementation rather > >> than optimal semantics. So it's an uncomfortable proposal to change it > >> now. But what other way to fix this for existing users? What other way > >> to make the interface more intuitive for future users? And what could > >> break by removing it now? I guess somebody who already subtracts Shmem > >> from Cached. > >> > >> What are your thoughts on this? > > > > My thoughts are NAK. A misleading stat is not so bad as a > > misleading stat whose meaning we change in some random kernel. > > > > By all means improve Documentation/filesystems/proc.txt on Cached. > > By all means promote Active(file)+Inactive(file)-Buffers as often a > > better measure (though Buffers itself is obscure to me - is it intended > > usually to approximate resident FS metadata?). By all means work on > > /proc/meminfo-v2 (though that may entail dispiritingly long discussions). > > > > We have to assume that Cached has been useful to some people, and that > > they've learnt to subtract Shmem from it, if slow or no swap concerns them. > > > > Added Konstantin to Cc: he's had valuable experience of people learning > > to adapt to the numbers that we put out. > > > > I think everything will ok. Subtraction of shmem isn't widespread practice, > more like secret knowledge. This wasn't documented and people who use > this should be aware that this might stop working at any time. So, ACK. I'll take your ACK as cancelling my NAK then; but I do still remain uncomfortable with such a change - I think "we" would do much better to add fields with the necessary missing information to /proc/meminfo, than mess around with the meaning of existing fields. But if I'm the only one who thinks that way, ignore me. Hugh -- 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>