On Mon, Dec 14, 2020 at 10:36:29AM +0100, Laurent Dufour wrote: > Le 14/12/2020 à 03:03, Joel Fernandes a écrit : > > On Tue, Jul 07, 2020 at 01:31:37PM +0800, Chinwen Chang wrote: > > [..] > > > > > Hi Laurent, > > > > > > > > > > We merged SPF v11 and some patches from v12 into our platforms. After > > > > > several experiments, we observed SPF has obvious improvements on the > > > > > launch time of applications, especially for those high-TLP ones, > > > > > > > > > > # launch time of applications(s): > > > > > > > > > > package version w/ SPF w/o SPF improve(%) > > > > > ------------------------------------------------------------------ > > > > > Baidu maps 10.13.3 0.887 0.98 9.49 > > > > > Taobao 8.4.0.35 1.227 1.293 5.10 > > > > > Meituan 9.12.401 1.107 1.543 28.26 > > > > > WeChat 7.0.3 2.353 2.68 12.20 > > > > > Honor of Kings 1.43.1.6 6.63 6.713 1.24 > > > > > > > > That's great news, thanks for reporting this! > > > > > > > > > > > > > > By the way, we have verified our platforms with those patches and > > > > > achieved the goal of mass production. > > > > > > > > Another good news! > > > > For my information, what is your targeted hardware? > > > > > > > > Cheers, > > > > Laurent. > > > > > > Hi Laurent, > > > > > > Our targeted hardware belongs to ARM64 multi-core series. > > > > Hello! > > > > I was trying to develop an intuition about why does SPF give improvement for > > you on small CPU systems. This is just a high-level theory but: > > > > 1. Assume the improvement is because of elimination of "blocking" on > > mmap_sem. > > Could it be that the mmap_sem is acquired in write-mode unnecessarily in some > > places, thus causing blocking on mmap_sem in other paths? If so, is it > > feasible to convert such usages to acquiring them in read-mode? > > That's correct, and the goal of this series is to try not holding the > mmap_sem in read mode during page fault processing. > > Converting mmap_sem holder from write to read mode is not so easy and that > work as already been done in some places. If you think there are areas where > this could be done, you're welcome to send patches fixing that. > > > 2. Assume the improvement is because of lesser read-side contention on > > mmap_sem. > > On small CPU systems, I would not expect reducing cache-line bouncing to give > > such a dramatic improvement in performance as you are seeing. > > I don't think cache line bouncing reduction is the main sourcec of > performance improvement, I would rather think this is the lower part here. > I guess this is mainly because during loading time a lot of page fault is > occuring and thus SPF is reducing the contention on the mmap_sem. Thanks for the reply. I think I also wrongly assumed that acquiring mmap rwsem in write mode in a syscall makes SPF moot. Peter explained to me on IRC that tere's still perf improvement in write mode if an unrelated VMA is modified while another VMA is faulting. CMIIW - not an mm expert by any stretch. Thanks! - Joel
