On Fri, Oct 29, 2010 at 06:22:54PM +0300, Andriy Gapon wrote: > 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. Let me repeat it once more: zfs does not properly caches the vnode data content in the page cache (the cache is used in a weaker sence, not meaning the freebsd 'cached' memory, but a cache in more common sence). Not doing the optimization I mentioned would mean always allocating the pages and making it (partially) valid for each read call. > > > 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? Filling the cache with an appropriate content.
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