Monday, September 7, 2015

Asynchronous and synchronous I/O in Android NDK

This week, I had some issues on my tests in a Lollipop device with AOSP. I really wanted to have synchronous reads/writes so that I could evaluate my solution in a different way. I am also trying to minimize the effects of caching as much as possible. If you check for async and sync I/O operations for Linux, you'll a lot of references to the flags O_DIRECT, O_SYNC, O_DSYNC, ...

Actually, that's a good way to implement. However, with Android NDK apps, things are not so straightforward.

First, to have a good understanding about the O_*SYNC flags, check this link. Of course, the man page for the command open as well.

First, O_DIRECT does not work in Android since 4.4. See this link for more details about it.  That's sad. So, let's try to use the O_*SYNC data.

O_SYNC and O_DSYNC work fine in Android. But, as the description say, only for writes. Another detail: for Android, O_SYNC has the same semantics as O_DSYNC. That's good, but I still want something similar to reads as well.

Why don't we use O_RSYNC? Well, Android does not implement it :-( But it's not the only one... there are other Linux distributions that don't do it either.

What about dropping caches?? See this link for more details. Hum, that works, but after the first read, the data will be cached again :-(

So, I am still looking for a solution for Android Lollipop. Hope to post it soon!


Joella Koonce said...
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Martin Jurek said...

Please did you find a solution for your problem?
I am fighting with same problem.
Please help me if u can.