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SubjectRe: [RFC PATCH 0/4] Gang scheduling in CFS
On 12/25/2011 12:58 PM, Avi Kivity wrote:
> On 12/23/2011 12:36 PM, Ingo Molnar wrote:
> > * Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> wrote:
> >
> > > Here some interesting perf reports from inside the guest:
> > >
> > > Baseline:
> > > 29.79% ebizzy [kernel.kallsyms] [k] native_flush_tlb_others
> > > 18.70% ebizzy libc-2.12.so [.] __GI_memcpy
> > > 7.23% ebizzy [kernel.kallsyms] [k] get_page_from_freelist
> > > 5.38% ebizzy [kernel.kallsyms] [k] __do_page_fault
> > > 4.50% ebizzy [kernel.kallsyms] [k] ____pagevec_lru_add
> > > 3.58% ebizzy [kernel.kallsyms] [k] default_send_IPI_mask_logical
> > > 3.26% ebizzy [kernel.kallsyms] [k] native_flush_tlb_single
> > > 2.82% ebizzy [kernel.kallsyms] [k] handle_pte_fault
> > > 2.16% ebizzy [kernel.kallsyms] [k] kunmap_atomic
> > > 2.10% ebizzy [kernel.kallsyms] [k] _spin_unlock_irqrestore
> > > 1.90% ebizzy [kernel.kallsyms] [k] down_read_trylock
> > > 1.65% ebizzy [kernel.kallsyms] [k] __mem_cgroup_commit_charge.clone.4
> > > 1.60% ebizzy [kernel.kallsyms] [k] up_read
> > > 1.24% ebizzy [kernel.kallsyms] [k] __alloc_pages_nodemask
> > >
> > > Gang:
> > > 22.53% ebizzy libc-2.12.so [.] __GI_memcpy
> > > 9.73% ebizzy [kernel.kallsyms] [k] ____pagevec_lru_add
> > > 8.22% ebizzy [kernel.kallsyms] [k] get_page_from_freelist
> > > 7.80% ebizzy [kernel.kallsyms] [k] default_send_IPI_mask_logical
> > > 7.68% ebizzy [kernel.kallsyms] [k] native_flush_tlb_others
> > > 6.22% ebizzy [kernel.kallsyms] [k] __do_page_fault
> > > 5.54% ebizzy [kernel.kallsyms] [k] native_flush_tlb_single
> > > 4.44% ebizzy [kernel.kallsyms] [k] _spin_unlock_irqrestore
> > > 2.90% ebizzy [kernel.kallsyms] [k] kunmap_atomic
> > > 2.78% ebizzy [kernel.kallsyms] [k] __mem_cgroup_commit_charge.clone.4
> > > 2.76% ebizzy [kernel.kallsyms] [k] handle_pte_fault
> > > 2.16% ebizzy [kernel.kallsyms] [k] __mem_cgroup_uncharge_common
> > > 1.59% ebizzy [kernel.kallsyms] [k] down_read_trylock
> > > 1.43% ebizzy [kernel.kallsyms] [k] up_read
> > >
> > > I see the main difference between both the reports is:
> > > native_flush_tlb_others.
> >
> > So it would be important to figure out why ebizzy gets into so
> > many TLB flushes and why gang scheduling makes it go away.

<snip>

> The first part appears to be unrelated to ebizzy itself - it's the
> kunmap_atomic() flushing ptes. It could be eliminated by switching to a
> non-highmem kernel, or by allocating more PTEs for kmap_atomic() and
> batching the flush.

Um, that makes no sense. I was reading the profile as if it was a
backtrace.

Anyway, google says the ebizzy does a lot of large allocations - and
presumably deallocations - to simulate a database workload, which
explain the large number of tlb flushes.

--
error compiling committee.c: too many arguments to function



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