Re: barriers vs. reads

[Werner Almesberger <wa@almesberger.net>]

Jens Axboe wrote:
> If there are lots of barrier writes, you mean?

If there are a lot of writes before the barrier. Then the reads
after the barrier have to wait for all these writes to complete.
Of course, things get even worse if you have a lot of barriers,
in addition to there being lots of writes.

> To me, it's the expected behaviour. If you issue a barrier write, a read
> issued later should not be able to fetch old data.

... which pretty much kills the idea of short predictable
queuing delays :-(

> Hmm? Recently this was moved into __elv_add_request()

Ah, okay, different definition of where the elevator starts ;-)
Yes, I saw that.

BTW, in what cases would ELEVATOR_INSERT_FRONT combined with
a barrier make sense ?

- Werner

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