Messages in this thread | | | Date | Mon, 2 Jun 2008 01:00:40 +0200 | From | Pavel Machek <> | Subject | Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks? |
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On Sun 2008-06-01 15:47:36, Andrew Morton wrote: > On Mon, 2 Jun 2008 00:22:02 +0200 Pavel Machek <pavel@suse.cz> wrote:
Hi!
> > > I sense a strangeness. What are you actually trying to do with all of this? > > > > Okay, so I have around 400MB of data, I want it compressed, optionally > > encrypted and written to partition. > > > > Now, if I do it "naturally", I do writes, followed by fsync. > > > > That's bad, because kernel does not start write out immediately, and > > we waste time with idle disk. (If data compress really well, or > > encryption is off, this is significant). > > > > So we improve on this, by doing sync_file_range(SYNC_FILE_RANGE_WRITE) > > periodically. That keeps the disk busy, but occassionaly blocks the > > cpu... wasting time (which mostly hurts in compression+encryption > > case). > > yep. That's another use of sync_file_range(): to allow smart userspace > to optimise the kernel's IO scheduling decisions. > > > So... how can I keep _both_ cpu and disk busy? > > pthread_create() ;)
Actually it is easy enough to do with fork(), but...
> How about this: > > - Add a new SYNC_FILE_RANGE_NON_BLOCKING > > - If userspace set that flag, turn on writeback_control.nonblocking > in __filemap_fdatawrite_range(). > > - test it a lot.
Works for me. Is the expectation that I code this? I can certainly provide testing ;-).
> It will be userspace's responsibility to avoid burning huge amounts of > CPU repeatedly calling sync_file_range() and having it not actually write > anything.
Ok... I guess doing 10x sync_file_range() when writing 400MB of data is not excessive? Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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