Solaris Internals: Solaris 10 and OpenSolaris Kernel Architecture (2nd Edition)

9.3. Tracing the VM System

There are only a few DTrace probes in the VM system at the time of writing specifically those via the sysinfo provider and those in the page_create() code-path. It is, however, possible to trace a larger portion of the VM, using the fbt provider.

sol10# ./gvm.sh >vm.d sol10# ./vm.d <pid> sol10# more vm.d :::BEGIN { start = timestamp; } syscall::: /$target == pid/ { trace((timestamp - start) / 1000); } ::add_physmem:, ::sptcreate:, ... ::sptdestroy:, ::va_to_pfn: /$target == pid/ { trace((timestamp - start) / 1000); }

Running the VM trace script on a target process allows you to observe the VM level tasks of a target process. A simple example might be to trace the entire code path for a specific VM operation, however this often results in too many probes, creating significant probe effect. A simple script allows us to instrument just the VM system, by converting the function prototypes from the VM header files and auto-generating a DTrace script. This technique instruments just the perimeters of the VM modules we care aboutaddress spaces, segments, and page level interfaces.

0 => munmap 3194 0 -> as_unmap 3199 0 -> as_findseg 3206 0 <- as_findseg 3209 0 -> segvn_unmap 3211 0 -> segvn_lockop 3217 0 <- segvn_lockop 3219 0 -> hat_unload_callback 3221 0 -> page_get_pagesize 3236 0 <- page_get_pagesize 3237 0 -> hat_page_setattr 3239 0 <- hat_page_setattr 3240 0 -> free_vp_pages 3247 0 -> page_share_cnt 3252 0 -> hat_page_getshare 3255 0 <- hat_page_getshare 3256 0 <- page_share_cnt 3258 0 <- free_vp_pages 3259 0 <- hat_unload_callback 3261 0 -> seg_free 3263 0 -> as_removeseg 3265 0 <- as_removeseg 3270 0 -> segvn_free 3272 0 -> segvn_lockop 3273 0 <- segvn_lockop 3274 0 <- segvn_free 3279 0 <- seg_free 3281 0 <- segvn_unmap 3282 0 <- as_unmap 3284 0 <= munmap 3286

In this example, we can see the munmap() system call's implementationa munmap() request calls into the AS layer to locate a mapping for a given address, which searches the AVL tree for the target address space, locating all the mappings within the specified addresses. For each mapping, the AS layer calls the appropriate segment driver, which contains the majority of the unmap implementation for this address range. In this example, the seg_vn driver calls into the HAT to unload the MMU mappings for each of the pages within the supplied addresses.