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

2. 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

3. 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.

4. 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)

5. 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

6. Experiment Power test in DBT-3 Results
VMM: Xen 3.0.0
OS: Linux
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

7. 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