On Fri, Feb 19, 2016 at 9:40 AM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> 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. Actually, NR_FILE_PAGES could try to retire after that. Where only few places where it is used and looks like it's easy to replace it with something else, even more accurate. -- 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>