KernelForNote4The C10K problem Help save the best Linux news source on the web subscribe to Linux Weekly News Its time for web servers to handle ten thousand clients. Brendans blog File System Latency part 4. This is part 4 on file system latency, a series on storage IO performance from the application perspective see part 1, part 2 and part 3. In the previous post I showed how to trace file system latency from within My. SQL using the pid provider. In this post Ill show how similar data can be retrieved using the DTrace syscall and fbt providers. These allow us to trace at the system call layer, and deeper in the kernel at both the Virtual File System VFS interface and the specific file system itself. Syscall Tracing. By tracing at the system call layer, the file system can be traced system wide examining all applications simultaneously no p PID. NtlpWwgTBqg/hqdefault.jpg' alt='Kernel For Note 4' title='Kernel For Note 4' />1. Introduction. First of all note that these are not libc system calls, but real system calls provided by Linux Kernel. List is intended to cover Linux 2. File System Latency part 4. This is part 4 on file system latency, a series on storage IO performance from the application perspective see part 1, part 2 and part 3. How to Compile the Linux Kernel. The Linux kernel is the heart of any Linux system. It handles user inputoutput, hardware, and controlling the power in the computer. DTrace has the syscall provider for tracing system calls. Application. mysqld. Half Life Opposing Force Update Patch. HERE system calls. File System. V. Another advantage is that the syscalls are well understood and documented in the man pages. They are also much less likely to change than the mysql functions traced earlier I would say will never change until I saw the exec family of syscalls change slightly during the Solaris 1. To demonstrate syscall tracing, this DTrace script shows the latency of reads to ZFS by application name. Distribution count. Distribution count. New Advanced Uninstaller Special Edition 4 - And Full Version here. As seen previously with mysqldpidfslatency. DRAM. The slowest seen above reached only the 1. Tracing syscalls has been made dramatically easier with the introduction of the fds array, which allows file descriptor numbers to be converted into descriptive details, such as the file system type. Kernel For Note 4' title='Kernel For Note 4' />The array is indexed by file descriptor number, which for the read syscall is the first argument readfd, buf, size. Here the fifs file system member is checked on line 4, to only match reads to ZFS. This script can be modified to include other syscall types, and other file systems. See fsrwtime. d from the DTrace book for a version that matches more syscall types, and prints latency by file system, operation and mount point. Syscall analysis with DTrace is easy and effective. When doing amazing things by tracing the internals of an application, it can be easy to forget that syscall tracing may be good enough and a lot simpler. For this reason we put it early in the Strategy section of the File System chapter of the DTrace book. Drawbacks of the syscall approach are Cant currently execute in a Solaris zone or Joyent Smart. Machine only because the fds array isnt currently available the syscall provider does work in those environments. No query context. Expressing file system latency as a portion of query latency as was done with mysqldpidfslatencyslowlog. Unless this is inferred from syscall activity, such as via socket related syscalls it might be possible I havent tried yet. Stack Fishing. Another use of the syscall provider is to investigate how applications are using the file system the calling stack trace. This approach is how I initially found the above functions from the mysql source myread, mywrite, etc, which took me to the right place to start reading the code. You can also try this approach if the mysqldpidfslatency. First, make sure that the PID really is mysqld. Then, you can use stack fishing to find out what is being called instead of osfileread in that case. This one liner demonstrates the approach, frequency counting the syscall type and user stack frames for the given process calling into the ZFS file system. Z1. 5innobasecommit. P3. THDb0x. 11. 6. Kernel For Note 4' title='Kernel For Note 4' />Z1. P3. THDb0x. 95. mysqldZ1. P3. THDb0x. 13. 6. Z9endtrans. P3. THD2. Z2. 1mysqlexecutecommand. P3. THD0x. 21. 72. Z1. 1mysqlparse. P3. THDPKcj. PS20x. Z1. P3. THDPcj0xfc. 1. Z1. 0docommand. P3. THD0xb. 8. mysqldhandleoneconnection0x. ZN9. MYSQLLOG1. 4flushandsync. Ev0x. 2a. mysqldZN9. MYSQLLOG5write. EP3. THDP1. 1stiocache. P9. Logevent0x. Z1. P3. THDP1. 1stiocache. P9. Logevent0x. ZN9. MYSQLLOG7logxid. EP3. THDy0x. 51. Z1. 5hacommittrans. P3. THDb0x. 24a. Z9endtrans. P3. THD2. Z2. 1mysqlexecutecommand. P3. THD0x. 21. 72. Z1. 1mysqlparse. P3. THDPKcj. PS20x. Z1. P3. THDPcj0xfc. 1. Z1. 0docommand. P3. THD0xb. 8. mysqldhandleoneconnection0x. ZN9. Logevent. 14readlogevent. EP1. 1stiocache. P6. String. P1. 4pthreadmutex0xf. Z1. 7mysqlbinlogsend. P3. THDPcyt0x. 5dc. Z1. 6dispatchcommand. P3. THDPcj0xc. 09. Z1. 0docommand. P3. THD0xb. 8. mysqldhandleoneconnection0x. ZN9. Logevent. 14readlogevent. EP1. 1stiocache. P6. String. P1. 4pthreadmutex0xf. Z1. 7mysqlbinlogsend. P3. THDPcyt0x. 35e. Z1. 6dispatchcommand. P3. THDPcj0xc. 09. Z1. 0docommand. P3. THD0xb. 8. mysqldhandleoneconnection0x. The DTrace scripts shown earlier take the file system functions as seen in the above stack traces and measure latency. There are a lot more functions that DTrace can also inspect any of the lines above, along with the function entry arguments and return values. Stack traces show functions that can be individually traced with DTrace. Note that this one liner includes all file system IO, not just those during a query. The very first stack trace looks like it is as asynchronous database thread srvmasterthread trxpurge, whereas all the rest look like they are indeed during a query handleoneconnection docommand. The numbers at the bottom of the stack shows the number of times that entire stack trace was responsible for the syscall being called, during tracing I let it run for several seconds. VFS Tracing. Apart from the application itself and system calls, DTrace can also drill down into the kernel. The first location of interest is the Virtual File System abstraction layer, which is an interface that file systems are called from. Such a common interface is good fodder for DTracing. There isnt a vfs provider for DTrace at least, one that exposes the latency of events, and so to do this we can currently use the fbt provider to trace kernel internals. Introducing VFS into our IO stack. Application. mysqld. HERE VFS. File. Systems. ZFS UFS. V V. Advantages of tracing at the VFS level include All file system types can be traced from one place. The VFS interface functions are more stable than other parts of the kernel. Kernel context is available, including more information than fds makes available. You can find examples of VFS tracing in Chapter 5 of the DTrace book, which can be downloaded as a sample chapter PDF. Here is an example, solvfssnoop. VFS IO on Solaris. TIMEms UID PID PROCESS CALL KB PATH. Ive had to redact the filename info replaced portions with xxxxx, but you should still get the picture. This has all the useful details except for the latency, which can be added to the script by tracing the return probe as well as the entry probes, and comparing timestamps similar to how the syscalls were traced earlier. Since VFS IO can be very frequent thousands of IO per second, when I invoked the script above I added an action to exit after 1. The script also accepts a process name as an argument, eg, mysqld to only trace VFS IO from mysqld processes. Every day. Every hour. For some students living the dorm life at UK, the end of November brings with it a rush to find somewhere to stay during Thanksgiving break. Fourteen of the schools residence halls will close at 1. Wednesday, Nov. 2. Sunday, Nov. 2. 6.