Messages in this thread | | | Date | Mon, 6 Feb 2006 13:50:36 +0100 | From | Jens Axboe <> | Subject | Re: Which is simpler? (Was Re: [Suspend2-devel] Re: [ 00/10] [Suspend2] Modules support.) |
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On Mon, Feb 06 2006, Pavel Machek wrote: > > > interface for userland, and BTW going async will not give you too much > > > of performance advantage here... > > > > How do you know that? Suspend2 has async I/O, and can write the image as > > fast as the drive can take it. Some testing I did a while ago showed a max > > throughput of 16MB/s for swsusp vs 35 (what the drive is capable of) for > > Suspend2. Add LZF compression and it's 70MB/s vs 16MB/s. > > Userspace is perfectly capable of saturating I/O subsystem. No magical > "async io" is needed for that. time dd if=/dev/zero of=/dev/hda > bs=... if you don't believe me.
But the in-kernel swsusp stuff (looking at the current linus kernel now) does write each page to the swap in a sync manner. That definitely is pretty slow, I would not be surprised if Nigels numbers are correct. If you repeated without write back caching enabled, a factor 10 in slowdown as compared to async io would not surprise me.
Why don't you split the writeout into a few seperate loops? Step 1, submission, step 2, wait for completion, step 3, check for io errors. You could even collapse step 2+3 if you wanted, the important bit is to queue all the io first.
> > > Current uswsusp is 3K lines of code in kernel, 1K lines of code in > > > userspace. > > > > > > When we are done, we'll have perhaps 2.5K lines of code in kernel > > > (in-kernel swap writing support goes away), and maybe 20K lines in > > > userspace. > > > > Without adding which out of async I/O, compression, encryption, swap file > > support, and ordinary file support? > > I'll get same bandwidth as you, without need for async I/O. Async I/O > is not really a feature, suspend speed is. (There are existing > interfaces for doing AIO from userspace, anyway, but I'm pretty sure > they will not be needed
If you keep writing single pages sync, you sure as hell wont get anywhere near async io in speed...
-- Jens Axboe
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