Am 01.08.2018 um 19:59 schrieb Marek Olšák: > On Wed, Aug 1, 2018 at 1:52 PM, Christian König > <christian.koenig at amd.com> wrote: >> Am 01.08.2018 um 19:39 schrieb Marek Olšák: >>> On Wed, Aug 1, 2018 at 2:32 AM, Christian König >>> <christian.koenig at amd.com> wrote: >>>> Am 01.08.2018 um 00:07 schrieb Marek Olšák: >>>>> Can this be implemented as a wrapper on top of libdrm? So that the >>>>> tree (or hash table) isn't created for UMDs that don't need it. >>>> >>>> No, the problem is that an application gets a CPU pointer from one API >>>> and >>>> tries to import that pointer into another one. >>>> >>>> In other words we need to implement this independent of the UMD who >>>> mapped >>>> the BO. >>> Yeah, it could be an optional feature of libdrm, and other components >>> should be able to disable it to remove the overhead. >> >> The overhead is negligible, the real problem is the memory footprint. >> >> A brief look at the hash implementation in libdrm showed that this is >> actually really inefficient. >> >> I think we have the choice of implementing a r/b tree to map the CPU pointer >> addresses or implement a quadratic tree to map the handles. >> >> The later is easy to do and would also allow to get rid of the hash table as >> well. > We can also use the hash table from mesa/src/util. > > I don't think the overhead would be negligible. It would be a log(n) > insertion in bo_map and a log(n) deletion in bo_unmap. If you did > bo_map+bo_unmap 10000 times, would it be negligible? Compared to what the kernel needs to do for updating the page tables it is less than 1% of the total work. The real question is if it wouldn't be simpler to use a tree for the handles. Since the handles are dense you can just use an unbalanced tree which is really easy. For a tree of the CPU mappings we would need an r/b interval tree, which is hard to implement and quite some overkill. Do you have any numbers how many BOs really get a CPU mapping in a real world application? Christian. > > Marek
