On May 21, 2019 1:41:20 PM GMT+02:00, Minchan Kim <minc...@kernel.org> wrote: >On Tue, May 21, 2019 at 01:30:32PM +0200, Christian Brauner wrote: >> On Tue, May 21, 2019 at 08:05:52PM +0900, Minchan Kim wrote: >> > 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. >> >> It's a whole new API to address processes. We already have >> clone(CLONE_PIDFD) and pidfd_send_signal() as you have seen since you >> exported pidfd_to_pid(). And we're going to have pidfd_open(). Your >> syscall works only with pidfds so it's tied to this api as well so it >> should follow the naming scheme. This also makes life easier for >> userspace and is consistent. > >Okay. I will change the API name at next revision. >Thanks.
Thanks! Fwiw, there's been a similar patch by Oleksandr for pidfd_madvise I stumbled upon a few days back: https://gitlab.com/post-factum/pf-kernel/commit/0595f874a53fa898739ac315ddf208554d9dc897 He wanted to be cc'ed but I forgot. Christian