Dynamically reallocating memory in an on-demand code execution system

Systems and methods are described for dynamically adjusting quantities of computing resources allocated to virtual machine instances in an on-demand code execution system. An on-demand code execution system may execute user-submitted code on virtual machine instances, which may be provisioned with quantities of various computing resources (memory, storage, processor time, etc.). Users may request that code be executed on virtual machine instances having a particular quantity of a particular computing resource, and a previously provisioned virtual machine instance have a different quantity of the resource than the quantity requested. A resource reclamation system may thus be used to dynamically adjust the quantity of computing resources without reprovisioning the virtual machine instance. The resource reclamation system may execute a resource reclamation process within the virtual machine instance, which may claim or release a portion of the provisioned computing resource to cause the virtual machine instance to have the requested quantity.