Date: Fri, 29 Oct 2010 18:22:54 +0300 From: Andriy Gapon <avg@icyb.net.ua> To: Kostik Belousov <kostikbel@gmail.com> Cc: freebsd-stable@freebsd.org, Alexander Zagrebin <alexz@visp.ru> Subject: Re: 8.1-STABLE: zfs and sendfile: problem still exists Message-ID: <4CCAE6CE.9020509@icyb.net.ua> In-Reply-To: <20101029151727.GY2392@deviant.kiev.zoral.com.ua> References: <4CCACDC0.7050802@icyb.net.ua> <1BDB4D1B02274CC8AA2DD5E68190CB5D@vosz.local> <20101029145349.GX2392@deviant.kiev.zoral.com.ua> <4CCAE2B6.1050906@icyb.net.ua> <20101029151727.GY2392@deviant.kiev.zoral.com.ua>
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on 29/10/2010 18:17 Kostik Belousov said the following: > On Fri, Oct 29, 2010 at 06:05:26PM +0300, Andriy Gapon wrote: >> on 29/10/2010 17:53 Kostik Belousov said the following: >>> Could it be the priming of the vm object pages content ? >> >> Sorry, not familiar with this term. >> Do you mean prepopulation of vm object with valid pages? >> >>> Due to double-buffering, and (possibly false) optimization to only >> >> What optimization? > On zfs vnode read, the page from the corresponding vm object is only > populated with the vnode data if the page already exists in the > object. Do you mean a specific type of read? For "normal" reads it's the other way around - if the page already exists and is valid, then we read from the page, not from ARC. > Not doing the optimization would be to allocate the page uncoditionally > on the read if not already present, and copy the data from ARC to the page. >> >>> perform double-buffering when vm object already has some data cached, >>> reads can prime vm object page list before file is mmapped or >>> sendfile-ed. >>> >> >> No double-buffering is done to optimize anything. Double-buffering >> is a consequence of having page cache and ARC. The special >> "double-buffering code" is to just handle that fact - e.g. making >> sure that VOP_READ reads data from page cache instead of ARC if it's >> possible that the data in them differs (i.e. page cache has more >> recent data). >> >> So, if I understood the term 'priming' correctly, no priming should >> ever occur. > The priming is done on the first call to VOP_READ() with the right > offset after the page is allocated. Again, what is priming? -- Andriy Gapon
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