On Tue, May 21, 2019 at 4:39 AM Christian Brauner <christ...@brauner.io> wrote:
>
> On Tue, May 21, 2019 at 01:30:29PM +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.
>
> This is at least my reasoning. I'm not going to make this a whole big
> pedantic argument. If people have really strong feelings about not using
> this prefix then fine. But if syscalls can be grouped together and have
> consistent naming this is always a big plus.
My hope has been that pidfd use becomes normalized enough that
prefixing "pidfd_" to pidfd-accepting system calls becomes redundant.
We write write(), not fd_write(), right? :-) pidfd_open() makes sense
because the primary purpose of this system call is to operate on a
pidfd, but I think process_madvise() is fine.
