Messages in this thread | | | Subject | Re: [PATCH v3 2/2] mm, thp: avoid unnecessary swapin in khugepaged | From | Vlastimil Babka <> | Date | Thu, 17 Mar 2016 12:07:44 +0100 |
| |
On 03/14/2016 10:40 PM, Ebru Akagunduz wrote: > Currently khugepaged makes swapin readahead to improve > THP collapse rate. This patch checks vm statistics > to avoid workload of swapin, if unnecessary. So that > when system under pressure, khugepaged won't consume > resources to swapin. > > The patch was tested with a test program that allocates > 800MB of memory, writes to it, and then sleeps. The system > was forced to swap out all. Afterwards, the test program > touches the area by writing, it skips a page in each > 20 pages of the area. When waiting to swapin readahead > left part of the test, the memory forced to be busy > doing page reclaim. There was enough free memory during > test, khugepaged did not swapin readahead due to business. > > Test results: > > After swapped out > ------------------------------------------------------------------- > | Anonymous | AnonHugePages | Swap | Fraction | > ------------------------------------------------------------------- > With patch | 206608 kB | 204800 kB | 593392 kB | %99 | > ------------------------------------------------------------------- > Without patch | 351308 kB | 350208 kB | 448692 kB | %99 | > ------------------------------------------------------------------- > > After swapped in (waiting 10 minutes) > ------------------------------------------------------------------- > | Anonymous | AnonHugePages | Swap | Fraction | > ------------------------------------------------------------------- > With patch | 551992 kB | 368640 kB | 248008 kB | %66 | > ------------------------------------------------------------------- > Without patch | 586816 kB | 464896 kB | 213184 kB | %79 | > ------------------------------------------------------------------- > > Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Looks like a step in a good direction. Still might be worthwile to also wait for the swapin to complete, and actually collapse immediately, no?
> --- > Changes in v2: > - Add reference to specify which patch fixed (Ebru Akagunduz)
The reference is again missing in v3.
> - Fix commit subject line (Ebru Akagunduz) > > Changes in v3: > - Remove default values of allocstall (Kirill A. Shutemov) > > mm/huge_memory.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 86e9666..67a398c 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -102,6 +102,7 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait); > */ > static unsigned int khugepaged_max_ptes_none __read_mostly; > static unsigned int khugepaged_max_ptes_swap __read_mostly; > +static unsigned long int allocstall;
"int" here is unnecessary
> > static int khugepaged(void *none); > static int khugepaged_slab_init(void); > @@ -2438,7 +2439,7 @@ static void collapse_huge_page(struct mm_struct *mm, > struct page *new_page; > spinlock_t *pmd_ptl, *pte_ptl; > int isolated = 0, result = 0; > - unsigned long hstart, hend; > + unsigned long hstart, hend, swap, curr_allocstall; > struct mem_cgroup *memcg; > unsigned long mmun_start; /* For mmu_notifiers */ > unsigned long mmun_end; /* For mmu_notifiers */ > @@ -2493,7 +2494,14 @@ static void collapse_huge_page(struct mm_struct *mm, > goto out; > } > > - __collapse_huge_page_swapin(mm, vma, address, pmd); > + swap = get_mm_counter(mm, MM_SWAPENTS); > + curr_allocstall = sum_vm_event(ALLOCSTALL); > + /* > + * When system under pressure, don't swapin readahead. > + * So that avoid unnecessary resource consuming. > + */ > + if (allocstall == curr_allocstall && swap != 0) > + __collapse_huge_page_swapin(mm, vma, address, pmd); > > anon_vma_lock_write(vma->anon_vma); > > @@ -2790,6 +2798,7 @@ skip: > VM_BUG_ON(khugepaged_scan.address < hstart || > khugepaged_scan.address + HPAGE_PMD_SIZE > > hend); > + allocstall = sum_vm_event(ALLOCSTALL);
Why here? Rik said in v2:
> Khugepaged stores the allocstall value when it goes to sleep, > and checks it before calling (or not) __collapse_huge_page_swapin.
But that's not true, this is not "when it goes to sleep". So AFAICS it only observes the allocstalls between starting to scan a single pmd, and trying to collapse the pmd. So the window is quite tiny especially compared to I/O speeds, and this will IMHO catch only really frequent stalls. Placing it really at "when it goes to sleep" sounds better.
> ret = khugepaged_scan_pmd(mm, vma, > khugepaged_scan.address, > hpage); >
| |