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Date: Thu, 12 Apr 2007 16:08:43 +1000 From: Nick Piggin <nickpiggin@...oo.com.au> To: Eric Dumazet <dada1@...mosbay.com> CC: Rik van Riel <riel@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, Ulrich Drepper <drepper@...hat.com> Subject: Re: [PATCH] make MADV_FREE lazily free memory Eric Dumazet wrote: > Rik van Riel a écrit : > >> Eric Dumazet wrote: >> >>> Rik van Riel a écrit : >>> >>>> Make it possible for applications to have the kernel free memory >>>> lazily. This reduces a repeated free/malloc cycle from freeing >>>> pages and allocating them, to just marking them freeable. If the >>>> application wants to reuse them before the kernel needs the memory, >>>> not even a page fault will happen. >> >> >>> I dont understand this last sentence. If not even a page fault >>> happens, how the kernel knows that the page was eventually reused by >>> the application, and should not be freed in case of memory pressure ? >> >> >> Before maybe freeing the page, the kernel checks the referenced >> and dirty bits of the page table entries mapping that page. >> >>> ptr = mmap(some space); >>> madvise(ptr, length, MADV_FREE); >>> /* kernel may free the pages */ >> >> >> All this call does is: >> - clear the accessed and dirty bits >> - move the page to the far end of the inactive list, >> where it will be the first to be reclaimed >> >>> sleep(10); >>> >>> /* what the application must do know before reusing space ? */ >>> memset(ptr, data, 10000); >>> /* kernel should not free ptr[0..10000] now */ >> >> >> Two things can happen here. >> >> If this program used the pages before the kernel needed >> them, the program will be reusing its old pages. > > > ah ok, this is because accessed/dirty bits are set by hardware and not a > page fault. No it isn't. > Is it true for all architectures ? No. -- SUSE Labs, Novell Inc. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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