Re: [LSF/MM TOPIC]swap improvements for fast SSD

[Minchan Kim <minchan@kernel.org>]

On Tue, Jan 22, 2013 at 02:53:41PM +0800, Shaohua Li wrote:
> Hi,
> 
> Because of high density, low power and low price, flash storage (SSD) is a good
> candidate to partially replace DRAM. A quick answer for this is using SSD as
> swap. But Linux swap is designed for slow hard disk storage. There are a lot of
> challenges to efficiently use SSD for swap:

Many of below item could be applied in in-memory swap like zram, zcache.

> 
> 1. Lock contentions (swap_lock, anon_vma mutex, swap address space lock)
> 2. TLB flush overhead. To reclaim one page, we need at least 2 TLB flush. This
> overhead is very high even in a normal 2-socket machine.
> 3. Better swap IO pattern. Both direct and kswapd page reclaim can do swap,
> which makes swap IO pattern is interleave. Block layer isn't always efficient
> to do request merge. Such IO pattern also makes swap prefetch hard.

Agreed.

> 4. Swap map scan overhead. Swap in-memory map scan scans an array, which is
> very inefficient, especially if swap storage is fast.

Agreed.

> 5. SSD related optimization, mainly discard support
> 6. Better swap prefetch algorithm. Besides item 3, sequentially accessed pages
> aren't always in LRU list adjacently, so page reclaim will not swap such pages
> in adjacent storage sectors. This makes swap prefetch hard.

One of problem is LRU churning and I wanted to try to fix it.
http://marc.info/?l=linux-mm&m=130978831028952&w=4

> 7. Alternative page reclaim policy to bias reclaiming anonymous page.
> Currently reclaim anonymous page is considering harder than reclaim file pages,
> so we bias reclaiming file pages. If there are high speed swap storage, we are
> considering doing swap more aggressively.

Yeb. We need it. I tried it with extending vm_swappiness to 200.

From: Minchan Kim <minchan@kernel.org>
Date: Mon, 3 Dec 2012 16:21:00 +0900
Subject: [PATCH] mm: increase swappiness to 200

We have thought swap out cost is very high but it's not true
if we use fast device like swap-over-zram. Nonetheless, we can
swap out 1:1 ratio of anon and page cache at most.
It's not enough to use swap device fully so we encounter OOM kill
while there are many free space in zram swap device. It's never
what we want.

This patch makes swap out aggressively.

Cc: Luigi Semenzato <semenzato@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 kernel/sysctl.c |    3 ++-
 mm/vmscan.c     |    6 ++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 693e0ed..f1dbd9d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -130,6 +130,7 @@ static int __maybe_unused two = 2;
 static int __maybe_unused three = 3;
 static unsigned long one_ul = 1;
 static int one_hundred = 100;
+extern int max_swappiness;
 #ifdef CONFIG_PRINTK
 static int ten_thousand = 10000;
 #endif
@@ -1157,7 +1158,7 @@ static struct ctl_table vm_table[] = {
                .mode           = 0644,
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = &zero,
-               .extra2         = &one_hundred,
+               .extra2         = &max_swappiness,
        },
 #ifdef CONFIG_HUGETLB_PAGE
        {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 53dcde9..64f3c21 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -53,6 +53,8 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/vmscan.h>
 
+int max_swappiness = 200;
+
 struct scan_control {
        /* Incremented by the number of inactive pages that were scanned */
        unsigned long nr_scanned;
@@ -1626,6 +1628,7 @@ static int vmscan_swappiness(struct scan_control *sc)
        return mem_cgroup_swappiness(sc->target_mem_cgroup);
 }
 
+
 /*
  * Determine how aggressively the anon and file LRU lists should be
  * scanned.  The relative value of each set of LRU lists is determined
@@ -1701,11 +1704,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
        }
 
        /*
-        * With swappiness at 100, anonymous and file have the same priority.
         * This scanning priority is essentially the inverse of IO cost.
         */
        anon_prio = vmscan_swappiness(sc);
-       file_prio = 200 - anon_prio;
+       file_prio = max_swappiness - anon_prio;
 
        /*
         * OK, so we have swap space and a fair amount of page cache
-- 
1.7.9.5

> 8. Huge page swap. Huge page swap can solve a lot of problems above, but both
> THP and hugetlbfs don't support swap.

Another items are indirection layers. Please read Rik's mail below.
Indirection layers could give many flexibility to backends and helpful
for defragmentation.

One of idea I am considering is that makes hierarchy swap devides,
NOT priority-based. I mean currently swap devices are used up by prioirty order.
It's not good fit if we use fast swap and slow swap at the same time.
I'd like to consume fast swap device (ex, in-memory swap) firstly, then
I want to migrate some of swap pages from fast swap to slow swap to
make room for fast swap. It could solve below concern.
In addition, buffering via in-memory swap could make big chunk which is aligned
to slow device's block size so migration speed from fast swap to slow swap
could be enhanced so wear out problem would go away, too.

Quote from last KS2012 - http://lwn.net/Articles/516538/
"Andrea Arcangeli was also concerned that the first pages to be evicted from
memory are, by definition of the LRU page order, the ones that are least likely
to be used in the future. These are the pages that should be going to secondary
storage and more frequently used pages should be going to zcache. As it stands,
zcache may fill up with no-longer-used pages and then the system continues to
move used pages from and to the disk."