Re: [PATCH v3] memcg: add memory.vmscan_stat

Johannes Weiner
2011-08-08T12:44:21Z

On Fri, Jul 22, 2011 at 05:15:40PM +0900, KAMEZAWA Hiroyuki wrote:
> [PATCH] add memory.vmscan_stat
>
> commit log of commit 0ae5e89 " memcg: count the soft_limit reclaim in..."
> says it adds scanning stats to memory.stat file. But it doesn't because
> we considered we needed to make a concensus for such new APIs.
>
> This patch is a trial to add memory.scan_stat. This shows
> - the number of scanned pages(total, anon, file)
> - the number of rotated pages(total, anon, file)
> - the number of freed pages(total, anon, file)
> - the number of elaplsed time (including sleep/pause time)
>
> for both of direct/soft reclaim.
>
> The biggest difference with oringinal Ying's one is that this file
> can be reset by some write, as
>
> # echo 0 ...../memory.scan_stat
>
> Example of output is here. This is a result after make -j 6 kernel
> under 300M limit.
>
> [kamezawa@bluextal ~]$ cat /cgroup/memory/A/memory.scan_stat
> [kamezawa@bluextal ~]$ cat /cgroup/memory/A/memory.vmscan_stat
> scanned_pages_by_limit 9471864
> scanned_anon_pages_by_limit 6640629
> scanned_file_pages_by_limit 2831235
> rotated_pages_by_limit 4243974
> rotated_anon_pages_by_limit 3971968
> rotated_file_pages_by_limit 272006
> freed_pages_by_limit 2318492
> freed_anon_pages_by_limit 962052
> freed_file_pages_by_limit 1356440
> elapsed_ns_by_limit 351386416101
> scanned_pages_by_system 0
> scanned_anon_pages_by_system 0
> scanned_file_pages_by_system 0
> rotated_pages_by_system 0
> rotated_anon_pages_by_system 0
> rotated_file_pages_by_system 0
> freed_pages_by_system 0
> freed_anon_pages_by_system 0
> freed_file_pages_by_system 0
> elapsed_ns_by_system 0
> scanned_pages_by_limit_under_hierarchy 9471864
> scanned_anon_pages_by_limit_under_hierarchy 6640629
> scanned_file_pages_by_limit_under_hierarchy 2831235
> rotated_pages_by_limit_under_hierarchy 4243974
> rotated_anon_pages_by_limit_under_hierarchy 3971968
> rotated_file_pages_by_limit_under_hierarchy 272006
> freed_pages_by_limit_under_hierarchy 2318492
> freed_anon_pages_by_limit_under_hierarchy 962052
> freed_file_pages_by_limit_under_hierarchy 1356440
> elapsed_ns_by_limit_under_hierarchy 351386416101
> scanned_pages_by_system_under_hierarchy 0
> scanned_anon_pages_by_system_under_hierarchy 0
> scanned_file_pages_by_system_under_hierarchy 0
> rotated_pages_by_system_under_hierarchy 0
> rotated_anon_pages_by_system_under_hierarchy 0
> rotated_file_pages_by_system_under_hierarchy 0
> freed_pages_by_system_under_hierarchy 0
> freed_anon_pages_by_system_under_hierarchy 0
> freed_file_pages_by_system_under_hierarchy 0
> elapsed_ns_by_system_under_hierarchy 0
>
> total_xxxx is for hierarchy management.
>
> This will be useful for further memcg developments and need to be
> developped before we do some complicated rework on LRU/softlimit
> management.
>
> This patch adds a new struct memcg_scanrecord into scan_control struct.
> sc->nr_scanned at el is not designed for exporting information. For example,
> nr_scanned is reset frequentrly and incremented +2 at scanning mapped pages.
>
> For avoiding complexity, I added a new param in scan_control which is for
> exporting scanning score.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> Changelog:
> - fixed the trigger for recording nr_freed in shrink_inactive_list()
> Changelog:
> - renamed as vmscan_stat
> - handle file/anon
> - added "rotated"
> - changed names of param in vmscan_stat.
> ---
> Documentation/cgroups/memory.txt | 85 +++++++++++++++++++
> include/linux/memcontrol.h | 19 ++++
> include/linux/swap.h | 6 -
> mm/memcontrol.c | 172 +++++++++++++++++++++++++++++++++++++--
> mm/vmscan.c | 39 +++++++-
> 5 files changed, 303 insertions(+), 18 deletions(-)
>
> Index: mmotm-0710/Documentation/cgroups/memory.txt
> ===================================================================
> --- mmotm-0710.orig/Documentation/cgroups/memory.txt
> +++ mmotm-0710/Documentation/cgroups/memory.txt
> @@ -380,7 +380,7 @@ will be charged as a new owner of it.
>
> 5.2 stat file
>
> -memory.stat file includes following statistics
> +5.2.1 memory.stat file includes following statistics
>
> # per-memory cgroup local status
> cache - # of bytes of page cache memory.
> @@ -438,6 +438,89 @@ Note:
> file_mapped is accounted only when the memory cgroup is owner of page
> cache.)
>
> +5.2.2 memory.vmscan_stat
> +
> +memory.vmscan_stat includes statistics information for memory scanning and
> +freeing, reclaiming. The statistics shows memory scanning information since
> +memory cgroup creation and can be reset to 0 by writing 0 as
> +
> + #echo 0 > ../memory.vmscan_stat
> +
> +This file contains following statistics.
> +
> +[param]_[file_or_anon]_pages_by_[reason]_[under_heararchy]
> +[param]_elapsed_ns_by_[reason]_[under_hierarchy]
> +
> +For example,
> +
> + scanned_file_pages_by_limit indicates the number of scanned
> + file pages at vmscan.
> +
> +Now, 3 parameters are supported
> +
> + scanned - the number of pages scanned by vmscan
> + rotated - the number of pages activated at vmscan
> + freed - the number of pages freed by vmscan
> +
> +If "rotated" is high against scanned/freed, the memcg seems busy.
> +
> +Now, 2 reason are supported
> +
> + limit - the memory cgroup's limit
> + system - global memory pressure + softlimit
> + (global memory pressure not under softlimit is not handled now)
> +
> +When under_hierarchy is added in the tail, the number indicates the
> +total memcg scan of its children and itself.

In your implementation, statistics are only accounted to the memcg
triggering the limit and the respectively scanned memcgs.

Consider the following setup:

A
/ \
B C
/
D

If D tries to charge but hits the limit of A, then B's hierarchy
counters do not reflect the reclaim activity resulting in D.

That's not consistent with how hierarchy counters usually operate, and
neither with how you documented it.

On a non-technical note: as Ying Han and I were the other two people
working on reclaim and statistics, it really irks me that neither of
us were CCd on this. Especially on such a controversial change.