Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure

From: Christian Ehrhardt
Date: Wed Apr 21 2010 - 03:35:54 EST



Christian Ehrhardt wrote:


Rik van Riel wrote:
On 04/20/2010 11:32 AM, Johannes Weiner wrote:

The idea is that it pans out on its own. If the workload changes, new
pages get activated and when that set grows too large, we start shrinking
it again.

Of course, right now this unscanned set is way too large and we can end
up wasting up to 50% of usable page cache on false active pages.

Thing is, changing workloads often change back.

Specifically, think of a desktop system that is doing
work for the user during the day and gets backed up
at night.

You do not want the backup to kick the working set
out of memory, because when the user returns in the
morning the desktop should come back quickly after
the screensaver is unlocked.

IMHO it is fine to prevent that nightly backup job from not being finished when the user arrives at morning because we didn't give him some more cache - and e.g. a 30 sec transition from/to both optimized states is fine.
But eventually I guess the point is that both behaviors are reasonable to achieve - depending on the users needs.

What we could do is combine all our thoughts we had so far:
a) Rik could create an experimental patch that excludes the in flight pages
b) Johannes could create one for his suggestion to "always scan active file pages but only deactivate them when the ratio is off and otherwise strip buffers of clean pages"
c) I would extend the patch from Johannes setting the ratio of active/inactive pages to be a userspace tunable

A first revision of patch c is attached.
I tested assigning different percentages, so far e.g. 50 really behave like before and 25 protects ~42M Buffers in my example which would match the intended behavior - see patch for more details.

Checkpatch and some basic function tests went fine.
While it may be not perfect yet, I think it is ready for feedback now.

a,b,a+b would then need to be tested if they achieve a better behavior.

c on the other hand would be a fine tunable to let administrators (knowing their workloads) or distributions (e.g. different values for Desktop/Server defaults) adapt their installations.

In theory a,b and c should work fine together in case we need all of them.

The big question is, what workload suffers from
having the inactive list at 50% of the page cache?

So far the only big problem we have seen is on a
very unbalanced virtual machine, with 256MB RAM
and 4 fast disks. The disks simply have more IO
in flight at once than what fits in the inactive
list.

Did I get you right that this means the write case - explaining why it is building up buffers to the 50% max?


Thinking about it I wondered for what these Buffers are protected.
If the intention to save these buffers is for reuse with similar loads I wonder why I "need" three iozones to build up the 85M in my case.

Buffers start at ~0, after iozone run 1 they are at ~35, then after #2 ~65 and after run #3 ~85.
Shouldn't that either allocate 85M for the first directly in case that much is needed for a single run - or if not the second and third run just "resuse" the 35M Buffers from the first run still held?

Note - "1 iozone run" means "iozone ... -i 0" which sequentially writes and then rewrites a 2Gb file on 16 disks in my current case.

looking forward especially to patch b as I'd really like to see a kernel able to win back these buffers if they are no more used for a longer period while still allowing to grow&protect them while needed.

--

GrÃsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
Subject: [PATCH] mm: make working set portion that is protected tunable

From: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>

In discussion with Rik van Riel and Joannes Weiner we came up that there are
cases that want the current "save 50%" for the working set all the time and
others that would benefit from protectig only a smaller amount.

Eventually no "carved in stone" in kernel ratio will match all use cases,
therefore this patch makes the value tunable via a /proc/sys/vm/ interface
named active_inactive_ratio.

Example configurations might be:
- 50% - like the current kernel
- 0% - like a kernel pre "56e49d21 vmscan: evict use-once pages first"
- x% - any other percentage to allow customizing the system to its needs.

Due to our experiments the suggested default in this patch is 25%, but if
preferred I'm fine keeping 50% and letting admins/distros adapt as needed.

Signed-off-by: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
---

[diffstat]

[diff]
Index: linux-2.6/Documentation/sysctl/vm.txt
===================================================================
--- linux-2.6.orig/Documentation/sysctl/vm.txt 2010-04-21 06:32:23.000000000 +0200
+++ linux-2.6/Documentation/sysctl/vm.txt 2010-04-21 07:24:35.000000000 +0200
@@ -18,6 +18,7 @@

Currently, these files are in /proc/sys/vm:

+- active_inactive_ratio
- block_dump
- dirty_background_bytes
- dirty_background_ratio
@@ -57,6 +58,15 @@

==============================================================

+active_inactive_ratio
+
+The kernel tries to protect the active working set. Therefore a portion of the
+file pages is protected, meaning they are omitted when eviting pages until this
+ratio is reached.
+This tunable represents that ratio in percent and specifies the protected part
+
+==============================================================
+
block_dump

block_dump enables block I/O debugging when set to a nonzero value. More
Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c 2010-04-21 06:33:43.000000000 +0200
+++ linux-2.6/kernel/sysctl.c 2010-04-21 07:26:35.000000000 +0200
@@ -1271,6 +1271,15 @@
.extra2 = &one,
},
#endif
+ {
+ .procname = "active_inactive_ratio",
+ .data = &sysctl_active_inactive_ratio,
+ .maxlen = sizeof(sysctl_active_inactive_ratio),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one_hundred,
+ },

/*
* NOTE: do not add new entries to this table unless you have read
Index: linux-2.6/mm/memcontrol.c
===================================================================
--- linux-2.6.orig/mm/memcontrol.c 2010-04-21 06:31:29.000000000 +0200
+++ linux-2.6/mm/memcontrol.c 2010-04-21 09:00:22.000000000 +0200
@@ -893,12 +893,12 @@
int mem_cgroup_inactive_file_is_low(struct mem_cgroup *memcg)
{
unsigned long active;
- unsigned long inactive;
+ unsigned long file;

- inactive = mem_cgroup_get_local_zonestat(memcg, LRU_INACTIVE_FILE);
active = mem_cgroup_get_local_zonestat(memcg, LRU_ACTIVE_FILE);
+ file = active + mem_cgroup_get_local_zonestat(memcg, LRU_INACTIVE_FILE);

- return (active > inactive);
+ return (active > file * sysctl_active_inactive_ratio / 100);
}

unsigned long mem_cgroup_zone_nr_pages(struct mem_cgroup *memcg,
Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c 2010-04-21 06:31:29.000000000 +0200
+++ linux-2.6/mm/vmscan.c 2010-04-21 09:00:13.000000000 +0200
@@ -1459,14 +1459,23 @@
return low;
}

+/*
+ * sysctl_active_inactive_ratio
+ *
+ * Defines the portion of file pages within the active working set is going to
+ * be protected. The value represents the percentage that will be protected.
+ */
+int sysctl_active_inactive_ratio __read_mostly = 25;
+
static int inactive_file_is_low_global(struct zone *zone)
{
- unsigned long active, inactive;
+ unsigned long active, file;

active = zone_page_state(zone, NR_ACTIVE_FILE);
- inactive = zone_page_state(zone, NR_INACTIVE_FILE);
+ file = active + zone_page_state(zone, NR_INACTIVE_FILE);
+
+ return (active > file * sysctl_active_inactive_ratio / 100);

- return (active > inactive);
}

/**
Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h 2010-04-21 09:02:37.000000000 +0200
+++ linux-2.6/include/linux/mm.h 2010-04-21 09:02:51.000000000 +0200
@@ -1467,5 +1467,7 @@

extern void dump_page(struct page *page);

+extern int sysctl_active_inactive_ratio;
+
#endif /* __KERNEL__ */
#endif /* _LINUX_MM_H */