Date: Wed, 22 Jan 2014 22:45:04 -0500 From: Garrett Wollman <wollman@freebsd.org> To: Navdeep Parhar <np@FreeBSD.org> Cc: net@freebsd.org Subject: Re: Use of contiguous physical memory in cxgbe driver Message-ID: <21216.36928.132606.318491@hergotha.csail.mit.edu> In-Reply-To: <52E06104.70600@FreeBSD.org> References: <21216.22944.314697.179039@hergotha.csail.mit.edu> <52E06104.70600@FreeBSD.org>
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<<On Wed, 22 Jan 2014 16:23:32 -0800, Navdeep Parhar <np@FreeBSD.org> said: > On 01/22/14 15:52, Garrett Wollman wrote: >> At this point everyone is well aware that requiring contiguous >> physical page when the hardware can do scatter-gather is a very bad >> idea. > I wouldn't put it this way. Using buffers with size > PAGE_SIZE has its > advantages. These advantages do not come close to balancing out the disadvantage of "your server eventually falls off the network due to physmem fragmentation, better hope you can reset it remotely because driving in to work at 3 AM sucks." If any free pages are available at all, the allocation of a 4k jumbo mbuf will succeed. A 9k jumbo mbuf requires three physically contiguous pages, and it's very, very easy for physical memory to get fragmented to the point where that is impossible. -GAWollman
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