Re: [patch v3] swap: virtual swap readahead

Johannes Weiner
2009-06-09T19:39:40Z

On Tue, Jun 09, 2009 at 09:01:28PM +0200, Johannes Weiner wrote:
> [resend with lists cc'd, sorry]

[and fixed Hugh's email. crap]

> Hi,
>
> here is a new iteration of the virtual swap readahead. Per Hugh's
> suggestion, I moved the pte collecting to the callsite and thus out
> ouf swap code. Unfortunately, I had to bound page_cluster due to an
> array of that many swap entries on the stack, but I think it is better
> to limit the cluster size to a sane maximum than using dynamic
> allocation for this purpose.
>
> Thanks all for the helpful suggestions. KAMEZAWA-san and Minchan, I
> didn't incorporate your ideas in this patch as I think they belong in
> a different one with their own justifications. I didn't ignore them.
>
> Hannes
>
> ---
> The current swap readahead implementation reads a physically
> contiguous group of swap slots around the faulting page to take
> advantage of the disk head's position and in the hope that the
> surrounding pages will be needed soon as well.
>
> This works as long as the physical swap slot order approximates the
> LRU order decently, otherwise it wastes memory and IO bandwidth to
> read in pages that are unlikely to be needed soon.
>
> However, the physical swap slot layout diverges from the LRU order
> with increasing swap activity, i.e. high memory pressure situations,
> and this is exactly the situation where swapin should not waste any
> memory or IO bandwidth as both are the most contended resources at
> this point.
>
> Another approximation for LRU-relation is the VMA order as groups of
> VMA-related pages are usually used together.
>
> This patch combines both the physical and the virtual hint to get a
> good approximation of pages that are sensible to read ahead.
>
> When both diverge, we either read unrelated data, seek heavily for
> related data, or, what this patch does, just decrease the readahead
> efforts.
>
> To achieve this, we have essentially two readahead windows of the same
> size: one spans the virtual, the other one the physical neighborhood
> of the faulting page. We only read where both areas overlap.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Cc: Andi Kleen <andi@firstfloor.org>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Minchan Kim <minchan.kim@gmail.com>
> ---
> include/linux/swap.h | 4 ++-
> kernel/sysctl.c | 7 ++++-
> mm/memory.c | 55 +++++++++++++++++++++++++++++++++++++++++
> mm/shmem.c | 4 +--
> mm/swap_state.c | 67 ++++++++++++++++++++++++++++++++++++++-------------
> 5 files changed, 116 insertions(+), 21 deletions(-)
>
> version 3:
> o move pte selection to callee (per Hugh)
> o limit ra ptes to one pmd entry to avoid multiple
> locking/mapping of highptes (per Hugh)
>
> version 2:
> o fall back to physical ra window for shmem
> o add documentation to the new ra algorithm (per Andrew)
>
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -327,27 +327,14 @@ struct page *read_swap_cache_async(swp_e
> return found_page;
> }
>
> -/**
> - * swapin_readahead - swap in pages in hope we need them soon
> - * @entry: swap entry of this memory
> - * @gfp_mask: memory allocation flags
> - * @vma: user vma this address belongs to
> - * @addr: target address for mempolicy
> - *
> - * Returns the struct page for entry and addr, after queueing swapin.
> - *
> +/*
> * Primitive swap readahead code. We simply read an aligned block of
> * (1 << page_cluster) entries in the swap area. This method is chosen
> * because it doesn't cost us any seek time. We also make sure to queue
> * the 'original' request together with the readahead ones...
> - *
> - * This has been extended to use the NUMA policies from the mm triggering
> - * the readahead.
> - *
> - * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
> */
> -struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> - struct vm_area_struct *vma, unsigned long addr)
> +static struct page *swapin_readahead_phys(swp_entry_t entry, gfp_t gfp_mask,
> + struct vm_area_struct *vma, unsigned long addr)
> {
> int nr_pages;
> struct page *page;
> @@ -373,3 +360,51 @@ struct page *swapin_readahead(swp_entry_
> lru_add_drain(); /* Push any new pages onto the LRU now */
> return read_swap_cache_async(entry, gfp_mask, vma, addr);
> }
> +
> +/**
> + * swapin_readahead - swap in pages in hope we need them soon
> + * @entry: swap entry of this memory
> + * @gfp_mask: memory allocation flags
> + * @vma: user vma this address belongs to
> + * @addr: target address for mempolicy
> + * @entries: swap slots to consider reading
> + * @nr_entries: number of @entries
> + * @cluster: readahead window size in swap slots
> + *
> + * Returns the struct page for entry and addr, after queueing swapin.
> + *
> + * This has been extended to use the NUMA policies from the mm
> + * triggering the readahead.
> + *
> + * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
> + */
> +struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> + struct vm_area_struct *vma, unsigned long addr,
> + swp_entry_t *entries, int nr_entries,
> + unsigned long cluster)
> +{
> + unsigned long pmin, pmax;
> + int i;
> +
> + if (!entries) /* XXX: shmem case */
> + return swapin_readahead_phys(entry, gfp_mask, vma, addr);
> + pmin = swp_offset(entry) & ~(cluster - 1);
> + pmax = pmin + cluster;
> + for (i = 0; i < nr_entries; i++) {
> + swp_entry_t swp = entries[i];
> + struct page *page;
> +
> + if (swp_type(swp) != swp_type(entry))
> + continue;
> + if (swp_offset(swp) > pmax)
> + continue;
> + if (swp_offset(swp) < pmin)
> + continue;
> + page = read_swap_cache_async(swp, gfp_mask, vma, addr);
> + if (!page)
> + break;
> + page_cache_release(page);
> + }
> + lru_add_drain(); /* Push any new pages onto the LRU now */
> + return read_swap_cache_async(entry, gfp_mask, vma, addr);
> +}
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -292,7 +292,9 @@ extern struct page *lookup_swap_cache(sw
> extern struct page *read_swap_cache_async(swp_entry_t, gfp_t,
> struct vm_area_struct *vma, unsigned long addr);
> extern struct page *swapin_readahead(swp_entry_t, gfp_t,
> - struct vm_area_struct *vma, unsigned long addr);
> + struct vm_area_struct *vma, unsigned long addr,
> + swp_entry_t *entries, int nr_entries,
> + unsigned long cluster);
>
> /* linux/mm/swapfile.c */
> extern long nr_swap_pages;
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2440,6 +2440,54 @@ int vmtruncate_range(struct inode *inode
> }
>
> /*
> + * The readahead window is the virtual area around the faulting page,
> + * where the physical proximity of the swap slots is taken into
> + * account as well in swapin_readahead().
> + *
> + * While the swap allocation algorithm tries to keep LRU-related pages
> + * together on the swap backing, it is not reliable on heavy thrashing
> + * systems where concurrent reclaimers allocate swap slots and/or most
> + * anonymous memory pages are already in swap cache.
> + *
> + * On the virtual side, subgroups of VMA-related pages are usually
> + * used together, which gives another hint to LRU relationship.
> + *
> + * By taking both aspects into account, we get a good approximation of
> + * which pages are sensible to read together with the faulting one.
> + */
> +static int swap_readahead_ptes(struct mm_struct *mm,
> + unsigned long addr, pmd_t *pmd,
> + swp_entry_t *entries,
> + unsigned long cluster)
> +{
> + unsigned long window, min, max, limit;
> + spinlock_t *ptl;
> + pte_t *ptep;
> + int i, nr;
> +
> + window = cluster << PAGE_SHIFT;
> + min = addr & ~(window - 1);
> + max = min + cluster;
> + /*
> + * To keep the locking/highpte mapping simple, stay
> + * within the PTE range of one PMD entry.
> + */
> + limit = addr & PMD_MASK;
> + if (limit > min)
> + min = limit;
> + limit = pmd_addr_end(addr, max);
> + if (limit < max)
> + max = limit;
> + limit = max - min;
> + ptep = pte_offset_map_lock(mm, pmd, min, &ptl);
> + for (i = nr = 0; i < limit; i++)
> + if (is_swap_pte(ptep[i]))
> + entries[nr++] = pte_to_swp_entry(ptep[i]);
> + pte_unmap_unlock(ptep, ptl);
> + return nr;
> +}
> +
> +/*
> * We enter with non-exclusive mmap_sem (to exclude vma changes,
> * but allow concurrent faults), and pte mapped but not yet locked.
> * We return with mmap_sem still held, but pte unmapped and unlocked.
> @@ -2466,9 +2514,14 @@ static int do_swap_page(struct mm_struct
> delayacct_set_flag(DELAYACCT_PF_SWAPIN);
> page = lookup_swap_cache(entry);
> if (!page) {
> + int nr, cluster = 1 << page_cluster;
> + swp_entry_t entries[cluster];
> +
> grab_swap_token(); /* Contend for token _before_ read-in */
> + nr = swap_readahead_ptes(mm, address, pmd, entries, cluster);
> page = swapin_readahead(entry,
> - GFP_HIGHUSER_MOVABLE, vma, address);
> + GFP_HIGHUSER_MOVABLE, vma, address,
> + entries, nr, cluster);
> if (!page) {
> /*
> * Back out if somebody else faulted in this pte
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1148,7 +1148,7 @@ static struct page *shmem_swapin(swp_ent
> pvma.vm_pgoff = idx;
> pvma.vm_ops = NULL;
> pvma.vm_policy = spol;
> - page = swapin_readahead(entry, gfp, &pvma, 0);
> + page = swapin_readahead(entry, gfp, &pvma, 0, NULL, 0, 0);
> return page;
> }
>
> @@ -1178,7 +1178,7 @@ static inline void shmem_show_mpol(struc
> static inline struct page *shmem_swapin(swp_entry_t entry, gfp_t gfp,
> struct shmem_inode_info *info, unsigned long idx)
> {
> - return swapin_readahead(entry, gfp, NULL, 0);
> + return swapin_readahead(entry, gfp, NULL, 0, NULL, 0, 0);
> }
>
> static inline struct page *shmem_alloc_page(gfp_t gfp,
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -112,6 +112,8 @@ static int min_percpu_pagelist_fract = 8
>
> static int ngroups_max = NGROUPS_MAX;
>
> +static int page_cluster_max = 5;
> +
> #ifdef CONFIG_MODULES
> extern char modprobe_path[];
> #endif
> @@ -966,7 +968,10 @@ static struct ctl_table vm_table[] = {
> .data = &page_cluster,
> .maxlen = sizeof(int),
> .mode = 0644,
> - .proc_handler = &proc_dointvec,
> + .proc_handler = &proc_dointvec_minmax,
> + .strategy = &sysctl_intvec,
> + .extra1 = &zero,
> + .extra2 = &page_cluster_max,
> },
> {
> .ctl_name = VM_DIRTY_BACKGROUND,
>
> --
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