Re: [RFC 0/7] introduce memory hinting API for external process

[Oleksandr Natalenko]

On Tue, May 21, 2019 at 02:04:00PM +0200, Christian Brauner wrote:
> On May 21, 2019 1:41:20 PM GMT+02:00, Minchan Kim <minchan@xxxxxxxxxx> 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 kernelâs 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.

Thanks :).

FWIW, since this submission is essentially a continuation of our discussion
involving my earlier KSM submissions here, I won't move my gitlab branch
forward and will be happy to assist with what we have here, be it
pidfd_madvise() or a set or /proc files (or smth else).

> 
> Christian
> 

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Senior Software Maintenance Engineer