On 11/09/2016 09:07 AM, Jens Axboe wrote:
On 11/09/2016 01:40 AM, Jan Kara wrote:
So for devices with write cache, you will completely drain the device
before waking anybody waiting to issue new requests. Isn't it too
strict?
In particular may_queue() will allow new writers to issue new writes
once
we drop below the limit so it can happen that some processes will be
effectively starved waiting in may_queue?
It is strict, and perhaps too strict. In testing, it's the only method
that's proven to keep the writeback caching devices in check. It will
round robin the writers, if we have more, which isn't necessarily a bad
thing. Each will get to do a burst of depth writes, then wait for a new
one.
Well, I'm more concerned about a situation where one writer does a
bursty write and blocks sleeping in may_queue(). Another writer
produces a steady flow of write requests so that never causes the
write queue to completely drain but that writer also never blocks in
may_queue() when it starts queueing after write queue has somewhat
drained because it never submits many requests in parallel. In such
case the first writer would get starved AFAIU.
I see what you are saying. I can modify the logic to ensure that if we
do have a waiter, we queue up others behind it. That should get rid of
that concern.
I added that - if we currently have a waiter, we'll add ourselves to the
back of the waitqueue and wait.
--
Jens Axboe
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