Re: [PATCH V4][for-next]mm: add a new vector based madvise syscall

On Wed, Feb 17, 2016 at 09:47:06AM -0800, Shaohua Li wrote:

On Tue, Feb 16, 2016 at 04:08:02PM -0800, Andrew Morton wrote:

quoted

On Thu, 10 Dec 2015 16:03:37 -0800 Shaohua Li [off-list ref] wrote:

quoted

In jemalloc, a free(3) doesn't immediately free the memory to OS even
the memory is page aligned/size, and hope the memory can be reused soon.
Later the virtual address becomes fragmented, and more and more free
memory are aggregated. If the free memory size is large, jemalloc uses
madvise(DONT_NEED) to actually free the memory back to OS.

The madvise has significantly overhead paritcularly because of TLB
flush. jemalloc does madvise for several virtual address space ranges
one time. Instead of calling madvise for each of the ranges, we
introduce a new syscall to purge memory for several ranges one time. In
this way, we can merge several TLB flush for the ranges to one big TLB
flush. This also reduce mmap_sem locking and kernel/userspace switching.

I'm running a simple memory allocation benchmark. 32 threads do random
malloc/free/realloc.

CPU count?  (Does that matter much?)

32. It does. the tlb flush overhead depends on the cpu count. 

quoted

quoted

Corresponding jemalloc patch to utilize this API is
attached.

No it isn't ;)

Sorry, I attached it in first post, but not this one. Attached is the
one I tested against this patch.

quoted

Who maintains jemalloc?  Are they signed up to actually apply the
patch?  It would be bad to add the patch to the kernel and then find
that the jemalloc maintainers choose not to use it!

Jason Evans (cced) is the author of jemalloc. I talked to him before, he
is very positive to this new syscall.

quoted

quoted

Without patch:
real    0m18.923s
user    1m11.819s
sys     7m44.626s
each cpu gets around 3000K/s TLB flush interrupt. Perf shows TLB flush
is hotest functions. mmap_sem read locking (because of page fault) is
also heavy.

with patch:
real    0m15.026s
user    0m48.548s
sys     6m41.153s
each cpu gets around 140k/s TLB flush interrupt. TLB flush isn't hot at
all. mmap_sem read locking (still because of page fault) becomes the
sole hot spot.

This is a somewhat underwhelming improvement, given that it's a
synthetic microbenchmark.

Yes, this test does malloc, free, calloc, realloc, so it doesn't only
benchmark the madvisev.

quoted

quoted

Another test malloc a bunch of memory in 48 threads, then all threads
free the memory. I measure the time of the memory free.
Without patch: 34.332s
With patch:    17.429s

This is more whelming.

Do we have a feel for how much benefit this patch will have for
real-world workloads?  That's pretty important.

Sure, we'll post some real-world data.

Hi Andrew,

Sorry I can't post real-world data. Our workloads used to suffer from
TLB flush overhead very much, but now looks something is changed, TLB
flush overhead isn't significant in the workloads.

Jemalloc guys (Dave, CCed) also made progress to improve jemalloc, they
can reduce TLB flush without kernel changes.

In the summary, the patch doesn't have benefit as expected in our real
workloads now. Unless somebody has other usage cases, I'd drop this
patch.

Thanks,
Shaohua