[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 2/4] xen: introduce grant_map_exists
On Mon, 6 Oct 2014, Jan Beulich wrote: > >>> On 06.10.14 at 15:46, <stefano.stabellini@xxxxxxxxxxxxx> wrote: > > On Mon, 6 Oct 2014, Jan Beulich wrote: > >> >>> On 06.10.14 at 15:08, <stefano.stabellini@xxxxxxxxxxxxx> wrote: > >> > The other approach to do this, that I avoided because it is slower (2 > >> > spinlocks instead of 1), is to use the existing mapcount function. > >> > mapcount is based on: > >> > > >> > for ( handle = 0; handle < lgt->maptrack_limit; handle++ ) > >> > > >> > do you think it is better? > >> > >> No, that's worse: This is the Dom0 side of things, and the loop > >> bound would still be controlled by that same command line option. > >> > >> So between the two, the original solution would be better, and > >> maybe enforcing a lower limit on DomU-s might actually be an > >> option. > > > > What about keeping track of the highest ref number actually used so far? > > I could use that as upper limit instead of the theoretical max. It would > > also make the execution faster. > > But the loops are already using a value on the order of the high > watermark, not the theoretical one. I.e. you're not alway looping > for a long time, it is just possible for this to happen. OK, I understand. The default max value is 32 pages that lead to 16384 iterations in grant_map_exists. Should I reduce it? Given the simple operations in the loop, the function shouldn't actually be so slow to be a security risk. It should take less that 0.1 milliseconds to complete. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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