Messages in this thread | | | Date | Wed, 12 Sep 2018 20:12:30 +0530 | From | Arun KS <> | Subject | Re: [RFC] memory_hotplug: Free pages as pageblock_order |
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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.
> >> > > >> > > If this looks okey, I'll modify users of set_online_page_callback >> > > and resend clean patch. >> > >> > [...] >> > >> > > +static int generic_online_pages(struct page *page, unsigned int order); >> > > +static online_pages_callback_t online_pages_callback = generic_online_pages; >> > > + >> > > +static int generic_online_pages(struct page *page, unsigned int order) >> > > +{ >> > > + unsigned long nr_pages = 1 << order; >> > > + struct page *p = page; >> > > + unsigned int loop; >> > > + >> > > + for (loop = 0 ; loop < nr_pages ; loop++, p++) { >> > > + __ClearPageReserved(p); >> > > + set_page_count(p, 0); > > btw. you want init_page_count here. Do you mean replace set_page_count(p, 0) with init_page_count(page)? Because init_page_count is setting the page _refcount to 1
static inline void init_page_count(struct page *page) { set_page_count(page, 1); }
I thought in case of higher order pages only the first struct page should have _refcount to 1 before calling __free_pages(). Please correct me if wrong.
Regards, Arun
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