tags
: Linux, syscalls, O
Usecases §
- Allocate
anon memory (malloc uses it internally)
- Create efficient read and write of files vs
read() and write()
- Glue some resource(portion of a file) to VAS of some process.
- Multiple processes can each map the same resource into their memory space
- IPC (
MAP_SHARED | MAP_ANONYMOUS)
- More control on permission
- Eliminates the protection domain crossing from system calls and
- kernel/userspace data copies that I/O system calls such as read() imply. ( See O )
Issues w using it in Databases §
Transactional Safety §
- OS can flush dirty pages at any time (even if the writing transaction hasn’t commited!)
- Best case scenario can be read-only
- Solution: OS CoW, Shadow paging
I/O Stalls §
- Because with
mmap, the OS manages your file I/O, you have no idea if your page is in memory or disk
- Reading from disk can cause I/O stalls
Error Handing §
- We want to validate checksum of a
page when it’s loaded to memory
- We want to check for corruption before writing things back to disk
- We cannot do these because we don’t have the access to that level when I/O is handled by the OS
- Trying to access mmap data can cause
SIGBUS
- Solution: Instead of having all these error handling in the
buffer pool module, now all this error handling can be seen in rest of your codebase
Random notes §
Precautionary Tales §
- Use memory maps if you access data randomly, sparse reads.
read files normally if you access data sequentially.
Interesting flags §
MAP_SHARED (Share w other processes)MAP_PRIVATE (Uses Copy on Write)MAP_ANONYMOUS
More resources §