presto-main/src/main/java/io/prestosql/operator/OperatorContext.java/OperatorContext/requestMemoryRevoking
class OperatorContext:    public long requestMemoryRevoking()
    {
        long revokedMemory = 0L;
        Runnable listener = null;
        synchronized (this) {
            if (!isMemoryRevokingRequested() && operatorMemoryContext.getRevocableMemory() > 0) {
                memoryRevokingRequested = true;
                revokedMemory = operatorMemoryContext.getRevocableMemory();
                listener = memoryRevocationRequestListener;
            }
        }
        if (listener != null) {
            runListener(listener);
        }
        return revokedMemory;
    }