What is the cheapest way to roundup() or ceil() to multiple of 1000?

For example if cheapest way to roundup is implemented in the ceil function:

function ceil(uint a, uint m) returns (uint) {
    //some cheap roundup to muliple function
}

then it is expected that:

• ceil(123456,1000) == 124000
• ceil(1,1000) == 1000
• ceil(0,1000) == 0
• ceil(123000,1000) == 123000