On 12/01/2016 11:16 AM, Linus Torvalds wrote:

On Thu, Dec 1, 2016 at 5:50 AM, Kent Overstreet
[off-list ref] wrote:

quoted

That said, I'm not sure how I feel about Jens's exact approach... it seems to me
that this can really just live within the writeback code, I don't know why it
should involve the block layer at all. plus, if I understand correctly his code
has the effect of blocking in generic_make_request() to throttle, which means
due to the way the writeback code is structured we'll be blocking with page
locks held.

Yeah, I do *not* believe that throttling at the block layer is at all
the right thing to do.

I do think that the block layer needs to throttle, but it needs to be
seen as a "last resort" kind of thing, where the block layer just
needs to limit how much it will have oending. But it should be seen as
a failure mode, not as a write balancing issue.

Because the real throttling absolutely needs to happen when things are
marked dirty, because no block layer throttling will ever fix the
situation where you just have too much memory dirtied that you cannot
free because it will take a minute to write out.

So throttling at a VM level is sane. Throttling at a block layer level is not.

It's two different kinds of throttling. The vm absolutely should
throttle at dirty time, to avoid having insane amounts of memory dirty.
On the block layer side, throttling is about avoid the device queues
being too long. It's very similar to the buffer bloating on the
networking side. The block layer throttling is not a fix for the vm
allowing too much memory to be dirty and causing issues, it's about
keeping the device response latencies in check.

-- 
Jens Axboe

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