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

[Simon Jeons <simon.jeons@gmail.com>]

On Wed, 2013-01-23 at 16:58 +0900, Minchan Kim wrote:
> 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

What's LRU history as you mentioned in your LRU churning patchset?

> 
> > 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."
> 
> From riel@redhat.com Sun Apr 10 17:50:10 2011
> Date: Sun, 10 Apr 2011 20:50:01 -0400
> From: Rik van Riel <riel@redhat.com>
> To: Linux Memory Management List <linux-mm@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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> 


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