Timo Sirainen put forth on 3/10/2010 3:19 PM: > On 10.8.2009, at 20.01, Timo Sirainen wrote: > >> (3.5. Implement async I/O filesystem backend.) > > You know what I found out today? Linux doesn't support async IO for regular > buffered files. I had heard there were issues, but I thought it was mainly > about some annoying APIs and such. Anyone know if some project has > successfully figured out some usable way to do async disk IO? The > possibilities seem to be: > > a) Use Linux's native AIO, which requires direct-io for files. This *might* > not be horribly bad for mail files. After all, same mail is rarely read > multiple times. Except when parsing its headers first and then its body. > Maybe the process could do some internal buffering?.. > > I guess no one ever tried my posix_fadvise() patch? The idea was that it > would tell the kernel after closing a mail file that it's no longer needed in > memory, so kernel could remove it from page cache. I never heard any positive > or negative comments about how it affected performance.. > http://dovecot.org/patches/1.1/fadvise.diff > > b) Use threads, either via some library or implement yourself. Each thread of > course uses some extra memory. Also enabling threads causes glibc to start > using a thread-safe version of malloc() (I think?), which slows things down > (unless that can be avoided, maybe by using clone() directly instead of > pthreads?). > > c) I read someone's idea about using posix_fadvise() and fincore() functions > to somehow make it "kind of work, usually, maybe". I'm not sure if there's a > practical way to make them work though. And of course I don't think fincore() > has even been accepted by Linus yet. >
Considering the extent to which Linus hates O_DIRECT, I would think if he was a fan of Async I/O at all, he'd have pushed its use via the buffer cache. Given that Async I/O is implemented via O_DIRECT I'd say Linus isn't a fan of Async I/O either. I've not read anything Linus has written on Async I/O, if he even has, I'm merely making an educated guess based on the current implementation of Async I/O in Linux. -- Stan
