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

[Christian Brauner]

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.

Christian