Re: [PATCH] mremap: add MREMAP_NOHOLE flag --resend

[Daniel Micay <danielmicay-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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> I didn't follow this thread. However, as you mentioned MADV_FREE will
> make many page fault, I jump into here.
> One of the benefit with MADV_FREE in current implementation is to
> avoid page fault as well as no zeroing.
> Why did you see many page fault?

I think I just misunderstood why it was still so much slower than not
using purging at all.

>> I get ~20k requests/s with jemalloc on the ebizzy benchmark with this
>> dual core ivy bridge laptop. It jumps to ~60k requests/s with MADV_FREE
>> IIRC, but disabling purging via MALLOC_CONF=lg_dirty_mult:-1 leads to
>> 3.5 *million* requests/s. It has a similar impact with TCMalloc.
> 
> When I tested MADV_FREE with ebizzy, I saw similar result two or three
> times fater than MADV_DONTNEED. But It's no free cost. It incurs MADV_FREE
> cost itself*(ie, enumerating all of page table in the range and clear
> dirty bit and tlb flush). Of course, it has mmap_sem with read-side lock.
> If you see great improve when you disable purging, I guess mainly it's
> caused by no lock of mmap_sem so some threads can allocate while other
> threads can do page fault. The reason I think so is I saw similar result
> when I implemented vrange syscall which hold mmap_sem read-side lock
> during very short time(ie, marking the volatile into vma, ie O(1) while
> MADV_FREE holds a lock during enumerating all of pages in the range, ie O(N))

It stops doing mmap after getting warmed up since it never unmaps so I
don't think mmap_sem is a contention issue. It could just be caused by
the cost of the system call itself and TLB flush. I found perf to be
fairly useless in identifying where the time was being spent.

It might be much more important to purge very large ranges in one go
with MADV_FREE. It's a different direction than the current compromises
forced by MADV_DONTNEED.


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