Preventing Performance Degradation on Operating System Reboots Kenichi Kourai Tokyo Institute of Technology

Motivation OS reboot is not avoidable Many bugs in OSes Simple method for software rejuvenation Performance degradation after OS reboot The file cache is lost Disk access conflicts between virtual machines (VMs) VM VM OS reboot is not avoidable because operating systems have many bugs. When an operating system crashes or slows down critically, it must be rebooted. Applying patches to fix bugs often needs OS reboot. Furthermore, a technique called software rejuvenation has been proposed to prevent problems by software aging, but the simple method for OS rejuvenation is also OS reboot. Just after an operating system is rebooted, its performance is degraded. The primary cause is that the file cache on memory is lost and the cache miss rate increases. In addition, recently, multiple servers are consolidated into one physical machine with virtual machines. Under such server consolidation, disk access caused by cache miss conflicts between virtual machines and degrades the performance each other. shared physical disk

Warm-cache Reboot A new reboot mechanism to prevent the performance degradation Enables an OS to reuse the file cache Preserves the integrity of the file cache Our claim The file cache does not need to be discarded if its integrity is preserved The purpose of OS reboot is to initialize its internal state or to update its components To prevent such performance degradation after OS reboot, we propose a new reboot mechanism called the warm-cache reboot. This mechanism enables an operating system to be rebooted without losing the file cache and to reuse the file cache. The warm-cache reboot also preserves the integrity of the file cache. Our claim is that the file cache doesn’t need to be discarded if its integrity is preserved. The purpose of OS reboot is to initialize its internal state like data structures or to update its components, not to erase the file cache.

Preserving the File Cache The warm-cache reboot preserves cache pages on memory during OS reboot The VMM allocates the same memory to VMs The OS maintains a cache-mapping table cache-mapping table The warm-cache reboot preserves cache pages on memory during OS reboot. To achieve this, the virtual machine monitor allocates the same physical memory to rebooted virtual machines. In addition, the operating system maintains a table called cache-mapping table. This table is a hash table whose keys are a tuple of a device number, an inode number, and a file offset, and whose value is a frame number assigned to a cache page. When the operating system is rebooted, it first obtains the address of the cache-mapping table from the VMM and reserves its memory area. Then, it reserves all cache pages by traversing the table. cache page VM OS virtual machine monitor (VMM)

Protecting the File Cache Cache pages are write-protected The VMM reads file blocks into a protected cache page The VMM maintains a reuse flag write-protected To prevent cache pages from being corrupted by OS crashes, they are write-protected by the OS. In our system, the VMM reads file blocks from a disk into a protected cache page so that the contents are not corrupted during the file read. A cache page is write-protected in a VM, but the VMM can access it. To preserve the integrity of the file cache, the VMM maintains a reuse flag for each cache page. When the VMM reads file blocks into a cache page, it sets the reuse flag for the page. When the OS modifies the page, it unprotects the page and the VMM resets its reuse flag. If the reuse flag is not set, the cache page is not reused after OS reboot. VM OS set a reuse flag read virtual machine monitor (VMM) disk

Experiment Power test in DBT-3 Results VMM: Xen 3.0.0 OS: Linux 2.6.12 CPU: dual-core Opteron x2 Memory:12 GB Disk: Ultra SCSI Power test in DBT-3 To examine the performance just after OS reboot Results Warm-cache reboot No performance degradation Normal reboot Degraded by 67% To examine the performance just after OS reboot, we performed the power test in the DBT-3 benchmark. DBT-3 is a benchmark tool for databases and its power test measures the performance of the read access to databases. We measured the performance twice before OS reboot and twice after OS reboot. This figure shows the results. The pink bar is the result for the warm-cache reboot. The performance is not degraded just after OS reboot by reusing the file cache. On the other hand, in the normal reboot illustrated by the red bar, the performance is degraded by 67% due to cache misses. before after

Conclusion We proposed the warm-cache reboot Preserves the file cache during OS reboot Protects cache pages using the VMM Related work Rio file cache [Chen et al.'96] Preserves dirty file cache for reliability Warm-VM reboot [Kourai et al. '07] Preserves VMs during VMM reboot