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Date: Wed, 19 Sep 2018 06:48:25 +0530 From: Arun KS <arunks@...eaurora.org> To: Michal Hocko <mhocko@...nel.org> Cc: Balbir Singh <bsingharora@...il.com>, akpm@...ux-foundation.org, dan.j.williams@...el.com, vbabka@...e.cz, pasha.tatashin@...cle.com, iamjoonsoo.kim@....com, osalvador@...e.de, malat@...ian.org, gregkh@...uxfoundation.org, yasu.isimatu@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, arunks.linux@...il.com, vinmenon@...eaurora.org Subject: Re: [RFC] memory_hotplug: Free pages as pageblock_order On 2018-09-14 14:40, Michal Hocko wrote: > On Wed 12-09-18 20:12:30, Arun KS wrote: >> On 2018-09-12 18:47, Michal Hocko wrote: >> > On Wed 12-09-18 22:57:43, Balbir Singh wrote: >> > > On Wed, Sep 12, 2018 at 12:38:53PM +0200, Michal Hocko wrote: >> > > > On Wed 12-09-18 14:56:45, Arun KS wrote: >> > > > > When free pages are done with pageblock_order, time spend on >> > > > > coalescing pages by buddy allocator can be reduced. With >> > > > > section size of 256MB, hot add latency of a single section >> > > > > shows improvement from 50-60 ms to less than 1 ms, hence >> > > > > improving the hot add latency by 60%. >> > > > >> > > > Where does the improvement come from? You are still doing the same >> > > > amount of work except that the number of callbacks is lower. Is this the >> > > > real source of 60% improvement? >> > > > >> > > >> > > It looks like only the first page of the pageblock is initialized, is >> > > some of the cost amortized in terms of doing one initialization for >> > > the page with order (order) and then relying on split_page and helpers >> > > to do the rest? Of course the number of callbacks reduce by a >> > > significant >> > > number as well. >> > >> > Ohh, I have missed that part. Now when re-reading I can see the reason >> > for the perf improvement. It is most likely the higher order free which >> > ends up being much cheaper. This part makes some sense. >> > >> > How much is this feasible is another question. Do not forget we have >> > those external providers of the online callback and those would need to >> > be updated as well. >> Sure Michal, I ll look into this. >> >> > >> > Btw. the normal memmap init code path does the same per-page free as >> > well. If we really want to speed the hotplug path then I guess the init >> > one would see a bigger improvement and those two should be in sync. >> Thanks for pointers, Will look further. > > I haven't looked closer and I will be travelling next week so just > hint. > Have a look at the nobootmem and how it frees pages to the page > allocator in __free_pages_boot_core. Seems exactly what you want and it > also answers your question about reference counting. Thanks Michal. Will send a new version after testing. Regards, Arun
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