Using high-resolution ( sub-kiloparsec scale ) data obtained by ALMA , we analyze the star formation rate ( SFR ) , gas content and kinematics in SDP 81 , a gravitationally-lensed starburst galaxy at redshift 3 . We estimate the SFR surface density ( \Sigma _ { \mathrm { SFR } } ) in the brightest clump of this galaxy to be 357 ^ { +135 } _ { -85 } \mbox { $ \mathrm { M } _ { \sun } $ } \mathrm { yr } ^ { -1 } \mathrm { kpc } ^ { % -2 } , over an area of 0.07 \pm 0.02 \mathrm { kpc } ^ { 2 } . Using the intensity-weighted velocity of CO ( 5-4 ) , we measure the turbulent velocity dispersion in the plane-of-the-sky and find \sigma _ { \mathrm { v,turb } } = 37 \pm 5 \mathrm { km s } ^ { -1 } for the clump , in good agreement with previous estimates along the line of sight , corrected for beam smearing . Our measurements of gas surface density , freefall time and turbulent Mach number allow us to compare the theoretical SFR from various star formation models with that observed , revealing that the role of turbulence is crucial to explaining the observed SFR in this clump . While the Kennicutt Schmidt ( KS ) relation predicts an SFR surface density of \Sigma _ { \mathrm { SFR,KS } } = 52 \pm 17 \mbox { $ \mathrm { M } _ { \sun } $ } \mathrm { yr } ^ { -1 % } \mathrm { kpc } ^ { -2 } , the single-freefall model by Krumholz , Dekel and McKee ( KDM ) predicts \Sigma _ { \mathrm { SFR,KDM } } = 106 \pm 37 \mbox { $ \mathrm { M } _ { \sun } $ } \mathrm { yr } ^ { % -1 } \mathrm { kpc } ^ { -2 } . In contrast , the multi-freefall ( turbulence ) model by Salim , Federrath and Kewley ( SFK ) gives \Sigma _ { \mathrm { SFR,SFK } } = 491 ^ { +139 } _ { -194 } \mbox { $ \mathrm { M } _ { \sun } $ } % \mathrm { yr } ^ { -1 } \mathrm { kpc } ^ { -2 } . Although the SFK relation overestimates the SFR in this clump ( possibly due to the negligence of magnetic fields ) , it provides the best prediction among the available models . Finally , we compare the star formation and gas properties of this galaxy to local star-forming regions and find that the SFK relation provides the best estimates of SFR in both local and high-redshift galaxies .