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SubjectRe: [patch v3] swap: virtual swap readahead
On Tue, 9 Jun 2009 21:37:02 +0200
Johannes Weiner <hannes@cmpxchg.org> wrote:

> 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;

Hmm, max = min + window ?

Thanks,
-Kame

> > + /*
> > + * 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,
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org. For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>



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