On Tue, May 21, 2019 at 10:42:00AM +0200, Christian Brauner wrote: > On Mon, May 20, 2019 at 12:52:47PM +0900, Minchan Kim wrote: > > - Background > > > > The Android terminology used for forking a new process and starting an app > > from scratch is a cold start, while resuming an existing app is a hot start. > > While we continually try to improve the performance of cold starts, hot > > starts will always be significantly less power hungry as well as faster so > > we are trying to make hot start more likely than cold start. > > > > To increase hot start, Android userspace manages the order that apps should > > be killed in a process called ActivityManagerService. ActivityManagerService > > tracks every Android app or service that the user could be interacting with > > at any time and translates that into a ranked list for lmkd(low memory > > killer daemon). They are likely to be killed by lmkd if the system has to > > reclaim memory. In that sense they are similar to entries in any other > > cache. > > Those apps are kept alive for opportunistic performance improvements but > > those performance improvements will vary based on the memory requirements of > > individual workloads. > > > > - Problem > > > > Naturally, cached apps were dominant consumers of memory on the system. > > However, they were not significant consumers of swap even though they are > > good candidate for swap. Under investigation, swapping out only begins > > once the low zone watermark is hit and kswapd wakes up, but the overall > > allocation rate in the system might trip lmkd thresholds and cause a cached > > process to be killed(we measured performance swapping out vs. zapping the > > memory by killing a process. Unsurprisingly, zapping is 10x times faster > > even though we use zram which is much faster than real storage) so kill > > from lmkd will often satisfy the high zone watermark, resulting in very > > few pages actually being moved to swap. > > > > - Approach > > > > The approach we chose was to use a new interface to allow userspace to > > proactively reclaim entire processes by leveraging platform information. > > This allowed us to bypass the inaccuracy of the kernel’s LRUs for pages > > that are known to be cold from userspace and to avoid races with lmkd > > by reclaiming apps as soon as they entered the cached state. Additionally, > > it could provide many chances for platform to use much information to > > optimize memory efficiency. > > > > IMHO we should spell it out that this patchset complements MADV_WONTNEED > > and MADV_FREE by adding non-destructive ways to gain some free memory > > space. MADV_COLD is similar to MADV_WONTNEED in a way that it hints the > > kernel that memory region is not currently needed and should be reclaimed > > immediately; MADV_COOL is similar to MADV_FREE in a way that it hints the > > kernel that memory region is not currently needed and should be reclaimed > > when memory pressure rises. > > > > To achieve the goal, the patchset introduce two new options for madvise. > > One is MADV_COOL which will deactive activated pages and the other is > > MADV_COLD which will reclaim private pages instantly. These new options > > complement MADV_DONTNEED and MADV_FREE by adding non-destructive ways to > > gain some free memory space. MADV_COLD is similar to MADV_DONTNEED in a way > > that it hints the kernel that memory region is not currently needed and > > should be reclaimed immediately; MADV_COOL is similar to MADV_FREE in a way > > that it hints the kernel that memory region is not currently needed and > > should be reclaimed when memory pressure rises. > > > > This approach is similar in spirit to madvise(MADV_WONTNEED), but the > > information required to make the reclaim decision is not known to the app. > > Instead, it is known to a centralized userspace daemon, and that daemon > > must be able to initiate reclaim on its own without any app involvement. > > To solve the concern, this patch introduces new syscall - > > > > struct pr_madvise_param { > > int size; > > const struct iovec *vec; > > } > > > > int process_madvise(int pidfd, ssize_t nr_elem, int *behavior, > > struct pr_madvise_param *restuls, > > struct pr_madvise_param *ranges, > > unsigned long flags); > > > > The syscall get pidfd to give hints to external process and provides > > pair of result/ranges vector arguments so that it could give several > > hints to each address range all at once. > > > > I guess others have different ideas about the naming of syscall and options > > so feel free to suggest better naming. > > Yes, all new syscalls making use of pidfds should be named > pidfd_<action>. So please make this pidfd_madvise.
I don't have any particular preference but just wondering why pidfd is so special to have it as prefix of system call name. > > Please make sure to Cc me on this in the future as I'm maintaining > pidfds. Would be great to have Jann on this too since he's been touching > both mm and parts of the pidfd stuff with me. Sure!