Minchan Kim | 23 Jan 08:58 2013

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

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 <at> 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 <at> google.com>
Signed-off-by: Minchan Kim <minchan <at> 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
 <at>  <at>  -130,6 +130,7  <at>  <at>  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
 <at>  <at>  -1157,7 +1158,7  <at>  <at>  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
 <at>  <at>  -53,6 +53,8  <at>  <at> 
 #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;
 <at>  <at>  -1626,6 +1628,7  <at>  <at>  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
 <at>  <at>  -1701,11 +1704,10  <at>  <at>  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."

From riel <at> redhat.com Sun Apr 10 17:50:10 2011
Date: Sun, 10 Apr 2011 20:50:01 -0400
From: Rik van Riel <riel <at> redhat.com>
To: Linux Memory Management List <linux-mm <at> kvack.org>
Subject: [LSF/Collab] swap cache redesign idea

On Thursday after LSF, Hugh, Minchan, Mel, Johannes and I were
sitting in the hallway talking about yet more VM things.

During that discussion, we came up with a way to redesign the
swap cache.  During my flight home, I came with ideas on how
to use that redesign, that may make the changes worthwhile.

Currently, the page table entries that have swapped out pages
associated with them contain a swap entry, pointing directly
at the swap device and swap slot containing the data. Meanwhile,
the swap count lives in a separate array.

The redesign we are considering moving the swap entry to the
page cache radix tree for the swapper_space and having the pte
contain only the offset into the swapper_space.  The swap count
info can also fit inside the swapper_space page cache radix
tree (at least on 64 bits - on 32 bits we may need to get
creative or accept a smaller max amount of swap space).

This extra layer of indirection allows us to do several things:

1) get rid of the virtual address scanning swapoff; instead
    we just swap the data in and mark the pages as present in
    the swapper_space radix tree

2) free swap entries as the are read in, without waiting for
    the process to fault it in - this may be useful for memory
    types that have a large erase block

3) together with the defragmentation from (2), we can always
    do writes in large aligned blocks - the extra indirection
    will make it relatively easy to have special backend code
    for different kinds of swap space, since all the state can
    now live in just one place

4) skip writeout of zero-filled pages - this can be a big help
    for KVM virtual machines running Windows, since Windows zeroes
    out free pages;   simply discarding a zero-filled page is not
    at all simple in the current VM, where we would have to iterate
    over all the ptes to free the swap entry before being able to
    free the swap cache page (I am not sure how that locking would
    even work)

    with the extra layer of indirection, the locking for this scheme
    can be trivial - either the faulting process gets the old page,
    or it gets a new one, either way it'll be zero filled

5) skip writeout of pages the guest has marked as free - same as
    above, with the same easier locking

Only one real question remaining - how do we handle the swap count
in the new scheme?  On 64 bit systems we have enough space in the
radix tree, on 32 bit systems maybe we'll have to start overflowing
into the "swap_count_continued" logic a little sooner than we are
now and reduce the maximum swap size a little?

> 
> I had some progresses in these areas recently:
> http://marc.info/?l=linux-mm&m=134665691021172&w=2
> http://marc.info/?l=linux-mm&m=135336039115191&w=2
> http://marc.info/?l=linux-mm&m=135882182225444&w=2
> http://marc.info/?l=linux-mm&m=135754636926984&w=2
> http://marc.info/?l=linux-mm&m=135754634526979&w=2
> But a lot of problems remain. I'd like to discuss the issues at the meeting.

I have an interest on this topic.
Thnaks.

> 
> Thanks,
> Shaohua
> 
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--

-- 
Kind regards,
Minchan Kim

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Gmane