Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

[Andrea Arcangeli]

On Fri, Dec 12, 2008 at 10:49:45AM -0600, Anthony Liguori wrote:
> I meant, if you wanted to pass a file descriptor as a raw device.  So:
>
> qemu -hda raw:fd=4
>
> Or something like that.  We don't support this today.

ah ok.

> I think bouncing the iov and just using pread/pwrite may be our best bet.  
> It means memory allocation but we can cap it.  Since we're using threads, 

It's already capped. However currently it generates an iovec, but
we've simply to check the iovcnt to be 1, if it's 1 we pread from
iov.iov_base, iov.iov_len. The dma api will take care to enforce
iovcnt to be 1 for the iovec if preadv/pwritev isn't detected at
compile time.

> we just can force a thread to sleep until memory becomes available so it's 
> actually pretty straight forward.

There's no way to detect that and wait for memory, it'd sigkill before
you can check... at least with the default overcommit. The way the dma
api works, is that it doesn't send a mega large writev, but send it in
pieces capped by the max buffer size, with many iovecs with iovcnt = 1.

> We can use libaio on older Linux's to simulate preadv/pwritev.  Use the 
> proper syscalls on newer kernels, on BSDs, and bounce everything else.

Given READV/WRITEV aren't available in not very recent kernels and
given that without O_DIRECT each iocb will become synchronous, we
can't use the libaio. Also once they fix linux-aio, if we do that, the
iocb logic would need to be largely refactored. So I'm not sure if it
worth it as it can't handle 2.6.16-18 when O_DIRECT is disabled (when
O_DIRECT is enabled we could just build an array of linear iocb).