We are developing an operating system, CEFOS, based on a fine-grain multi-threading model. By consisting a process with fine-grain threads, we can employ flexible scheduling schemes as long as thread dependencies allow. This paper presents WSC mechanism in CEFOS, which buffers multiple system-calls until the number of system-call requests or the time-slice reaches some threshold and then transfers the control to the operating system kernel with a bucket of the buffered system-call requests. We have developed CEFOS based on Linux and measured the system-call performance on commodity Intel processors. While fine-grained system-call scheduling with WSC caused frequent cache misses, it reduced context switches and improved the total execution time thanks to the reduction of TLB misses.