On Thu, 2009-02-12 at 00:44 -0500, John Moser wrote: > On 2/11/09, Daniel J Blueman <daniel.blue...@gmail.com> wrote: > > By modifying the boot-time readahead to be at lower I/O and processor > > priority than the boot scripts and asynchronous, I see a 20% reduction > > in overall boot time (from installing bootchart) on my desktop: 41s > > down to 33s. > > > A while back I pointed out that the file list for read-ahead was > sorted by block rather than boot sequence; and that by leaving it > sorted by boot sequence (i.e. first read == first need) and > parallelling it you would necessarily have exactly two states: 1) > waiting for a file readahead is trying to read anyway; 2) readahead is > ahead of you because the rest of the boot process has spent some time > CPU bound and not touching I/O. Also, it's likely as soon as you > enter state (2) that you stay there-- which is to say, almost > immediately. > Not true at all.
Remember that in the rotary disk case, it's not just I/O wait that kills you, but it's also the seek time of the disk. If the readahead list is sorted in need order, and run alongside other I/O activity, the disk is seeking all over the place. When sorted by block order, and run in one pass, the seek time should be eliminated. Again, this all depends on _your_ disk. Nobody other than Martin is following up with bootcharts[0] -- and his shows that parallel is a little worse then sync. The archives of ubuntu-devel will show my own investigation on this at the time. In summary: Sync is *always* better than no readahead at all. Parallel is *sometimes* better than no readahead, but in various cases is actually _worse_. When Parallel is not worse then no readahead, it is better than sync. Since the out-of-the-box has to work for everyone, I chose sync. If you have a very fast rotary disk (which Martin doesn't :p) or SSD, you definitely want parallel. But I'd then also recommend using sreadahead instead of readahead-list. I have some packages of sreadahead pending upload; but they require a kernel patch. Scott [0] though I should follow up with Martin OOB, because there's something very wrong with his machine ;) -- Scott James Remnant sc...@canonical.com
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