Messages in this thread | | | Date | Wed, 21 Mar 2012 14:33:31 +0000 | From | Attilio Rao <> | Subject | Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks |
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On 21/03/12 14:25, Stephan Diestelhorst wrote: > 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@citrix.com> >>>>> >>>>> 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. >
Yes, that's right, I just wanted to measure (possible) overhead in native Linux and the cost of leaving CONFIG_PARAVIRT_SPINLOCK on.
Thanks, Attilio
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