On Wed, Jun 19, 2019 at 02:27:50PM +0200, Michal Hocko wrote: > On Mon 10-06-19 20:12:47, Minchan Kim wrote: > > This patch is part of previous series: > > https://lore.kernel.org/lkml/20190531064313.193437-1-minchan@xxxxxxxxxx/T/#u > > Originally, it was created for external madvise hinting feature. > > > > https://lkml.org/lkml/2019/5/31/463 > > Michal wanted to separte the discussion from external hinting interface > > so this patchset includes only first part of my entire patchset > > > > - introduce MADV_COLD and MADV_PAGEOUT hint to madvise. > > > > However, I keep entire description for others for easier understanding > > why this kinds of hint was born. > > > > Thanks. > > > > This patchset is against on next-20190530. > > > > Below is description of previous entire patchset. > > ================= &< ===================== > > > > - 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. > > > > To achieve the goal, the patchset introduce two new options for madvise. > > One is MADV_COLD which will deactivate activated pages and the other is > > MADV_PAGEOUT 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_PAGEOUT 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_COLD 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 all is a very good background information suitable for the cover > letter. > > > 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; /* the size of this structure */ > > int cookie; /* reserved to support atomicity */ > > int nr_elem; /* count of below arrary fields */ > > int __user *hints; /* hints for each range */ > > /* to store result of each operation */ > > const struct iovec __user *results; > > /* input address ranges */ > > const struct iovec __user *ranges; > > }; > > > > int process_madvise(int pidfd, struct pr_madvise_param *u_param, > > unsigned long flags); > > But this and the following paragraphs are referring to the later step > when the madvise gains a remote process capabilities and that is out > of the scope of this patch series so I would simply remove it from > here. Andrew tends to put the cover letter into the first patch of the > series and that would be indeed > confusing here. Okay, I will remove the part in next revision.
