On 24 Nov 2016, at 18:59, Balbir Singh wrote:

On 23/11/16 03:25, Zi Yan wrote:

quoted

From: Zi Yan <redacted>

Hi all,

This patchset boosts the hugepage migration throughput and helps THP migration
which is added by Naoya's patches: https://lwn.net/Articles/705879/.

Motivation
===============================

In x86, 4KB page migrations are underutilizing the memory bandwidth compared
to 2MB THP migrations. I did some page migration benchmarking on a two-socket
Intel Xeon E5-2640v3 box, which has 23.4GB/s bandwidth, and discover
there are big throughput gap, ~3x, between 4KB and 2MB page migrations.

Here are the throughput numbers for different page sizes and page numbers:
        | 512 4KB pages | 1 2MB THP  |  1 4KB page
x86_64  |  0.98GB/s     |  2.97GB/s  |   0.06GB/s

As Linux currently use single-threaded page migration, the throughput is still
much lower than the hardware bandwidth, 2.97GB/s vs 23.4GB/s. So I parallelize
the copy_page() part of THP migration with workqueue and achieve 2.8x throughput.

Here are the throughput numbers of 2MB page migration:
           |  single-threaded   | 8-thread
x86_64 2MB |    2.97GB/s        | 8.58GB/s

Whats the impact on CPU utilization? Is there a huge impact?

Balbir Singh.

It depends on the throughput we can achieve.

For single-threaded copy, the current routine, it takes one CPU 2MB/(2.97GB/s) = 657.6 us
to copy one 2MB page.

For 8-thread copy, it take 8 CPUs 2MB/(8.58GB/s) = 227.6 us to copy one 2MB page.

If we have 8 idle CPUs, I think it worths using them.

I am going to add code to check idle_cpu() in the system before doing the copy.
If no idle CPUs are present, I can fall back to single-threaded copy.


--
Best Regards
Yan Zi