Jens Axboe wrote:
On Thu, Jun 19 2008, Matthew Wilcox wrote:
Use the noop elevator by default for drives that do not spin
[Not for applying]
SSDs do not benefit from the elevator. It just wastes precious CPU cycles.
By selecting the noop elevator by default, we can shave a few microseconds
off each IO.
I've brazenly stolen sd_vpd_inquiry from mkp's patch here:
http://marc.info/?l=linux-scsi&m=121264354724277&w=2
No need to have two copies of that ... but this will conflict with his code.
On to the self-criticism:
I don't intend the final version of this patch to include a printk for
the RPM or even a printk to say we switched IO elevator. I think we're
too verbose in SCSI as it is.
I think there's an opportunity to improve sd_vpd_inquiry() to remove
some of the duplicate code between sd_set_elevator() and sd_block_limits,
but it's not terribly important.
The switching of the elevators isn't particularly nice. I assume that
elevator_init("noop") cannot fail, which isn't true. It would be nice
to use the #if 0 block instead, but that causes a null ptr dereference
inside sysfs -- I suspect something isn't set up correctly.
I disagree with this approach. For now, lets just add a queue flag that
says the device doesn't have a seek penalty and let the io schedulers do
what they need to avoid that (it'd be a one-liner change to cfq and as).
There's more to io scheduling than just seek reduction, so this is the
wrong direction to take imo.
Very true - you still will get a significant win by coalescing IO's (say
for example, to do larger, aligned writes to flash devices).
ric
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