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Date: Wed, 4 Apr 2007 19:15:15 +0300 From: Dan Aloni <da-x@...atomic.org> To: Andrea Arcangeli <andrea@...e.de> Cc: Nick Piggin <npiggin@...e.de>, Hugh Dickins <hugh@...itas.com>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Linux Memory Management List <linux-mm@...ck.org>, tee@....com, holt@....com, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [rfc] no ZERO_PAGE? On Wed, Apr 04, 2007 at 05:27:17PM +0200, Andrea Arcangeli wrote: > On Wed, Apr 04, 2007 at 05:44:21PM +0300, Dan Aloni wrote: > > To refine that example, you could replace the file with a large anonymous > > memory pool and a lot of swap space committed to it. In that case - with > > no ZERO_PAGE, would the kernel needlessly swap-out the zeroed pages? > > Swapout or ram is the same in this context. The point is that it will > take 4k either in ram or swap, let's talk about virtual memory without > differentiating between ram or swap. The main difference is that disk-backed swap can create I/O pressure which would slow down the swap-outs that are not of zeroed pages (and other I/Os on that disk for that matter). For purely-RAM virtual memory the latency incured from managing newly allocated and zeroed pages is neglegible compared to the latencies you get from reading/flushing those pages to disk if you add swap to the picture. > > Perhaps it's an example too far-fetched to worth considering... > > Even if you would read the sparsed file to a malloced space (more > commonly that would be tmpfs) using the read syscall, those anon (or > tmpfs) pages would be _written_ first, which isn't the case we're > discussing here. > > You don't know what is on disk, so reading from disk (regardless of > what you read, holes, zeros or anything) provides useful information, > but you know what is in ram after an anon mmap: just zeros, reading > them can't provide useful information to any software. I agree. The swap I/O case still holds, though: swapping-in the zeroed pages that got swapped-out might incur unwanted overhead. -- Dan Aloni XIV LTD, http://www.xivstorage.com da-x (at) monatomic.org, dan (at) xiv.co.il - 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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