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Analyzer of MongoDB 2.4 's new feature returned ununderstandable results. The value of "resident" totally different from "pagesInMemory". But why ? -"resident" are coming from STAT. -"pageInMemory" are coming from mincore(). This slide illustrates this issue.
Citation preview
Why mincore() returns different
value of stat ?
mapped file
Interior of a process
FS
virtual mem
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mapped file
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page 256
physical mem
used by others
mmap() 1MB
mapped area
User-land Kernel-land
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
page 2
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page 256
physical mem
used by others
User-land Kernel-land
FS
virtual mem
page 1
mapped area
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page 256
page 1
mapped file
page 2
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touch
physical mem
used by others
User-land Kernel-land
Major page fault
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
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touch
physical mem
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used by others
Read page from disk
User-land Kernel-land
Major page fault
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
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touch
physical mem
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used by others
Associate physical memory with virtual memory
User-land Kernel-land
Major page fault
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
page 2
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page 256
touch
physical mem
page 2
page 3
used by others
User-land Kernel-land
FS
virtual mem
page 1
mapped area
page 2
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page 256
page 1
mapped file
page 2
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page 256
physical mem
page 2
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page 1
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used by others
User-land Kernel-land
swap out (Just image , not actual)
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
page 2
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page 256
touch
physical mem
page 2
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page 1
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used by others
User-land Kernel-land
FS
virtual mem
page 1
mapped area
page 2
page 3
page 4
page 5
page 6
page 7
page 256
page 1
mapped file
page 2
page 3
page 4
page 5
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page 7
page 256
touch
physical mem
page 2
page 3
page 6
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page 1
page 4
used by others
User-land Kernel-land
FS
virtual mem
page 1
mapped area
page 2
page 3
page 4
page 5
page 6
page 7
page 256
page 1
mapped file
page 2
page 3
page 4
page 5
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page 256
touch
physical mem
page 3
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page 1
page 4
used by others
User-land Kernel-land
FS
virtual mem
page 1
mapped area
page 2
page 3
page 4
page 5
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page 7
page 256
page 1
mapped file
page 2
page 3
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page 256
touch
physical mem
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page 1
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used by others
User-land Kernel-land
restart the process
FS
virtual mem
page 1
mapped area
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page 256
page 1
mapped file
page 2
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physical mem
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used by others
Old processUser-land Kernel-land
Nothing Kernel-land
FSpage 1
mapped file
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physical mem
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used by others
Kill process
User-land Kernel-land
FSpage 1
mapped file
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physical mem
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page 1
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used by others
virtual mem
New process
FS
virtual mem
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mapped area
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page 256
page 1
mapped file
page 2
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physical mem
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page 1
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used by others
mmap() 1MB
User-land Kernel-land
FS
virtual mem
page 1
mapped area
page 2
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page 256
page 1
mapped file
page 2
page 3
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page 256
physical mem
page 5
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page 1
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used by others
User-land Kernel-land
Actually, some pages are on memory
FS
virtual mem
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mapped area
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page 1
mapped file
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physical mem
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used by others
touch
ONLY associate physical memory with virtual memory
User-land Kernel-land
Minor page fault
names & seeing
FS
virtual mem
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mapped area
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page 1
mapped file
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physical mem
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used by others
PTE VMA
RES(resident)
SHR
mincore()
VIRT(virtual)
top
/proc/<pid>/smaps
/proc/<pid>/statm
:
User-land Kernel-land
Kernel code
fs/proc/task_mmu.c#L447
static void smaps_pte_entry(pte_t ptent, unsigned long addr, unsigned long ptent_size, struct mm_walk *walk){ struct mem_size_stats *mss = walk->private; struct vm_area_struct *vma = mss->vma; pgoff_t pgoff = linear_page_index(vma, addr); struct page *page = NULL; int mapcount;
if (pte_present(ptent)) { page = vm_normal_page(vma, addr, ptent); } else if (is_swap_pte(ptent)) { swp_entry_t swpent = pte_to_swp_entry(ptent);
if (!non_swap_entry(swpent)) mss->swap += ptent_size; else if (is_migration_entry(swpent)) page = migration_entry_to_page(swpent); } else if (pte_file(ptent)) { if (pte_to_pgoff(ptent) != pgoff) mss->nonlinear += ptent_size; }
if (!page) return;
if (PageAnon(page)) mss->anonymous += ptent_size;
if (page->index != pgoff) mss->nonlinear += ptent_size;
mss->resident += ptent_size; : :
mm/mincore.c#L108
static void mincore_pte_range(struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr, unsigned long end, unsigned char *vec){ unsigned long next; spinlock_t *ptl; pte_t *ptep;
ptep = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl); do { pte_t pte = *ptep; pgoff_t pgoff;
next = addr + PAGE_SIZE; if (pte_none(pte)) mincore_unmapped_range(vma, addr, next, vec); else if (pte_present(pte)) *vec = 1; else if (pte_file(pte)) { pgoff = pte_to_pgoff(pte); *vec = mincore_page(vma->vm_file->f_mapping, pgoff); } else { /* pte is a swap entry */ swp_entry_t entry = pte_to_swp_entry(pte); : :
