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Performing an eviction search can be very, very slow especially for a range restricted replacement. For example, a workload like gem_concurrent_blit will populate the entire GTT and then cause aperture thrashing. Since the GTT is a mix of active and inactive tiny objects, we have to search through almost 400k objects before finding anything inside the mappable region, and as this search is required before every operation performance falls off a cliff. Instead of performing the full search, we do a trial replacement of the node at a random location fitting the specified restrictions. We lose the strict LRU property of the GTT in exchange for avoiding the slow search (several orders of runtime improvement for gem_concurrent_blit 4KiB-global-gtt, e.g. from 5000s to 20s). The loss of LRU replacement is (later) mitigated firstly by only doing replacement if we find no freespace and secondly by execbuf doing a PIN_NONBLOCK search first before it starts thrashing (i.e. the random replacement will only occur from the already inactive set of objects). v2: Ascii-art, and check preconditionst v3: Rephrase final sentence in comment to explain why we don't bother with if (i915_is_ggtt(vm)) for preferring random replacement. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20170111112312.31493-3-chris@chris-wilson.co.uk
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.