On Tue, Jul 21, 2015 at 04:34:02PM -0700, Andrew Morton wrote: > On Sun, 19 Jul 2015 15:31:09 +0300 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote: > > > Hi, > > > > This patch set introduces a new user API for tracking user memory pages > > that have not been used for a given period of time. The purpose of this > > is to provide the userspace with the means of tracking a workload's > > working set, i.e. the set of pages that are actively used by the > > workload. Knowing the working set size can be useful for partitioning > > the system more efficiently, e.g. by tuning memory cgroup limits > > appropriately, or for job placement within a compute cluster. > > > > It is based on top of v4.2-rc2-mmotm-2015-07-15-16-46 > > It applies without conflicts to v4.2-rc2-mmotm-2015-07-17-16-04 as well > > > > ---- USE CASES ---- > > > > The unified cgroup hierarchy has memory.low and memory.high knobs, which > > are defined as the low and high boundaries for the workload working set > > size. However, the working set size of a workload may be unknown or > > change in time. With this patch set, one can periodically estimate the > > amount of memory unused by each cgroup and tune their memory.low and > > memory.high parameters accordingly, therefore optimizing the overall > > memory utilization. > > > > Another use case is balancing workloads within a compute cluster. > > Knowing how much memory is not really used by a workload unit may help > > take a more optimal decision when considering migrating the unit to > > another node within the cluster. > > > > Also, as noted by Minchan, this would be useful for per-process reclaim > > (https://lwn.net/Articles/545668/). With idle tracking, we could reclaim idle > > pages only by smart user memory manager. > > > > ---- USER API ---- > > > > The user API consists of two new proc files: > > > > * /proc/kpageidle. This file implements a bitmap where each bit corresponds > > to a page, indexed by PFN. > > What are the bit mappings? If I read the first byte of /proc/kpageidle > I get PFN #0 in bit zero of that byte? And the second byte of > /proc/kpageidle contains PFN #8 in its LSB, etc? The bit mapping is an array of u64 elements. Page at pfn #i corresponds to bit #i%64 of element #i/64. Byte order is native. Will add this to docs. > > Maybe this is covered in the documentation file. > > > When the bit is set, the corresponding page is > > idle. A page is considered idle if it has not been accessed since it was > > marked idle. > > Perhaps we can spell out in some detail what "accessed" means? I see > you've hooked into mark_page_accessed(), so a read from disk is an > access. What about a write to disk? And what about a page being > accessed from some random device (could hook into get_user_pages()?) Is > getting written to swap an access? When a dirty pagecache page is > written out by kswapd or direct reclaim? > > This also should be in the permanent documentation. OK, will add. > > > To mark a page idle one should set the bit corresponding to the > > page by writing to the file. A value written to the file is OR-ed with the > > current bitmap value. Only user memory pages can be marked idle, for other > > page types input is silently ignored. Writing to this file beyond max PFN > > results in the ENXIO error. Only available when CONFIG_IDLE_PAGE_TRACKING is > > set. > > > > This file can be used to estimate the amount of pages that are not > > used by a particular workload as follows: > > > > 1. mark all pages of interest idle by setting corresponding bits in the > > /proc/kpageidle bitmap > > 2. wait until the workload accesses its working set > > 3. read /proc/kpageidle and count the number of bits set > > Security implications. This interface could be used to learn about a > sensitive application by poking data at it and then observing its > memory access patterns. Perhaps this is why the proc files are > root-only (whcih I assume is sufficient). That's one point. Another point is that if we allow unprivileged users to access it, they may interfere with the system-wide daemon doing the regular scan and estimating the system wss. > Some words here about the security side of things and the reasoning > behind the chosen permissions would be good to have. > > > * /proc/kpagecgroup. This file contains a 64-bit inode number of the > > memory cgroup each page is charged to, indexed by PFN. > > Actually "closest online ancestor". This also should be in the > interface documentation. Actually, the userspace knows nothing about online/offline cgroups, because all cgroups used to be online and charge re-parenting was used to forcibly empty a memcg on deletion. Anyways, I'll add a note. > > > Only available when CONFIG_MEMCG is set. > > CONFIG_MEMCG and CONFIG_IDLE_PAGE_TRACKING I assume? No, it's present iff CONFIG_PROC_PAGE_MONITOR && CONFIG_MEMCG, because it might be useful even w/o CONFIG_IDLE_PAGE_TRACKING, e.g. in order to find out which memcg pages of a particular process are accounted to. > > > > > This file can be used to find all pages (including unmapped file > > pages) accounted to a particular cgroup. Using /proc/kpageidle, one > > can then estimate the cgroup working set size. > > > > For an example of using these files for estimating the amount of unused > > memory pages per each memory cgroup, please see the script attached > > below. > > Why were these put in /proc anyway? Rather than under /sys/fs/cgroup > somewhere? Presumably because /proc/kpageidle is useful in non-memcg > setups. Yes, one might use it for estimating active wss of a single process or the whole system. > > > ---- PERFORMANCE EVALUATION ---- > > "^___" means "end of changelog". Perhaps that should have been > "^---\n" - unclear. Sorry :-/ > > > Documentation/vm/pagemap.txt | 22 ++- > > I think we'll need quite a lot more than this to fully describe the > interface? Agree, the documentation sucks :-( Will try to forge something more thorough. Thanks, Vladimir -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
