Some TS methods such as TSRK do have an array of vectors like this to store the stage values. But not all TS methods have it. I am fine adding the scratch for TSTheta and any other method missing it. A little drawback is that it is used only by TSGetStages and the TSStep implementation does not necessarily need it. So I like the idea to return a temporary array with GetStages and free it with RestoreStages when the TS method does not have an array for stages internally. Hong On Jan 24, 2021, at 1:08 AM, Stefano Zampini <[email protected]<mailto:[email protected]>> wrote: You actually do not need a RestoreStages if you use a scratch Vec stages[2]; in TS_theta, and pass it back with TSGetStages_Theta. I understand PETSc philosophy is that every Get should have a Restore method, but is this really necessary for TSGetStages? Il giorno sab 23 gen 2021 alle ore 21:05 Zhang, Hong <[email protected]<mailto:[email protected]>> ha scritto: Done. Please check https://gitlab.com/petsc/petsc/-/merge_requests/3583 Sorry for any disturbance it caused. It was for the convenience of the adjoint implementation. The stages returned by TSGetStages_Theta currently do not reflect the true stages associated with these methods. The endpoint variant actually has two stages. This will be changed in a separate forthcoming MR, where TSRestoreStages() will be added and TSGetStages will return an array of vectors for the endpoint variant. Hong On Jan 21, 2021, at 4:16 AM, Stefano Zampini <[email protected]<mailto:[email protected]>> wrote: Hong, I do not understand why you changed the behavior of TSGetStages_Theta https://gitlab.com/petsc/petsc/-/merge_requests/3500/diffs#a582bbaec75f4ae14... with this MR https://gitlab.com/petsc/petsc/-/merge_requests/3500 Now, the non-endpoint variant does no longer use th->X as stage! Please revert this change Thanks -- Stefano -- Stefano