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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks
On Wednesday 21 March 2012, 13:49:28 Attilio Rao wrote: > On 21/03/12 13:22, Stephan Diestelhorst wrote: > > On Wednesday 21 March 2012, 13:04:25 Attilio Rao wrote: > > > >> On 21/03/12 10:20, Raghavendra K T wrote: > >> > >>> From: Jeremy Fitzhardinge<jeremy.fitzhardinge@xxxxxxxxxx> > >>> > >>> Rather than outright replacing the entire spinlock implementation in > >>> order to paravirtualize it, keep the ticket lock implementation but add > >>> a couple of pvops hooks on the slow patch (long spin on lock, unlocking > >>> a contended lock). > >>> > >>> Ticket locks have a number of nice properties, but they also have some > >>> surprising behaviours in virtual environments. They enforce a strict > >>> FIFO ordering on cpus trying to take a lock; however, if the hypervisor > >>> scheduler does not schedule the cpus in the correct order, the system can > >>> waste a huge amount of time spinning until the next cpu can take the lock. > >>> > >>> (See Thomas Friebel's talk "Prevent Guests from Spinning Around" > >>> http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.) > >>> > >>> To address this, we add two hooks: > >>> - __ticket_spin_lock which is called after the cpu has been > >>> spinning on the lock for a significant number of iterations but has > >>> failed to take the lock (presumably because the cpu holding the lock > >>> has been descheduled). The lock_spinning pvop is expected to block > >>> the cpu until it has been kicked by the current lock holder. > >>> - __ticket_spin_unlock, which on releasing a contended lock > >>> (there are more cpus with tail tickets), it looks to see if the next > >>> cpu is blocked and wakes it if so. > >>> > >>> When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub > >>> functions causes all the extra code to go away. > >>> > >>> > >> I've made some real world benchmarks based on this serie of patches > >> applied on top of a vanilla Linux-3.3-rc6 (commit > >> 4704fe65e55fb088fbcb1dc0b15ff7cc8bff3685), with both > >> CONFIG_PARAVIRT_SPINLOCK=y and n, which means essentially 4 versions > >> compared: > >> * vanilla - CONFIG_PARAVIRT_SPINLOCK - patch > >> * vanilla + CONFIG_PARAVIRT_SPINLOCK - patch > >> * vanilla - CONFIG_PARAVIRT_SPINLOCK + patch > >> * vanilla + CONFIG_PARAVIRT_SPINLOCK + patch > >> > >> > > [...] > > > >> == Results > >> This test points in the direction that Jeremy's rebased patches don't > >> introduce a peformance penalty at all, but also that we could likely > >> consider CONFIG_PARAVIRT_SPINLOCK option removal, or turn it on by > >> default and suggest disabling just on very old CPUs (assuming a > >> performance regression can be proven there). > >> > > Very interesting results, in particular knowing that in the one guest > > case things do not get (significantly) slower due to the added logic > > and LOCKed RMW in the unlock path. > > > > AFAICR, the problem really became apparent when running multiple guests > > time sharing the physical CPUs, i.e., two guests with eight vCPUs each > > on an eight core machine. Did you look at this setup with your tests? > > > > > > Please note that my tests are made on native Linux, without XEN involvement. > > You maybe meant that the spinlock paravirtualization became generally > useful in the case you mentioned? (2 guests, 8vpcu + 8vcpu)? Yes, that is what I meant. Just to clarify why you do not see any speed-ups, and were wondering why. If the whole point of the exercise was to see that there are no perforamnce regressions, fine. In that case I misunderstood. Stephan -- Stephan Diestelhorst, AMD Operating System Research Center stephan.diestelhorst@xxxxxxx Tel. +49 (0)351 448 356 719 Advanced Micro Devices GmbH Einsteinring 24 85609 Aschheim Germany Geschaeftsfuehrer: Alberto Bozzo Sitz: Dornach, Gemeinde Aschheim, Landkreis Muenchen Registergericht Muenchen, HRB Nr. 43632, WEEE-Reg-Nr: DE 12919551 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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