On 10/17/19 12:59 PM, Robert LeBlanc wrote:
On Thu, Oct 17, 2019 at 9:22 AM Casey Bodley <cbodley@xxxxxxxxxx> wrote:
With respect to this issue, civetweb and beast should behave the same.
Both frontends have a large thread pool, and their calls to
process_request() run synchronously (including blocking on rados
requests) on a frontend thread. So once there are more concurrent client
connections than there are frontend threads, new connections will block
until there's a thread available to service them.
Okay, this really helps me understand what's going on here. Is there
plans to remove the synchronous calls and make them async or improve
this flow a bit?
Absolutely yes, this work has been in progress for a long time now, and
octopus does get a lot of concurrency here. Eventually, all of
process_request() will be async-enabled, and we'll be able to run beast
with a much smaller thread pool.
Currently I'm seeing 1024 max concurrent ops and 512 thread pool. Does
this mean that on an equally distributed requests that one op could be
processing on the backend RADOS with another queued behind it waiting?
Is this done in round robin fashion so for 99% small io, a very long
RADOS request can get many IO blocked behind it because it is being
round-robin dispatched to the thread pool? (I assume the latter is
what I'm seeing).
rgw_max_concurrent_requests 1024
rgw_thread_pool_size 512
If I match the two, do you think it would help prevent small IO from
being blocked by larger IO?
rgw_max_concurrent_requests was added in support of the beast/async
work, precisely because (post-Nautilus) the number of beast threads will
no longer limit the number of concurrent requests. This variable is what
throttles incoming requests to prevent radosgw's resource consumption
from ballooning under heavy workload. And unlike the existing model
where a request remains in the queue until a thread is ready to service
it, any requests that exceed rgw_max_concurrent_requests will be
rejected with '503 SlowDown' in s3 or '498 Rate Limited' in swift.
With respect to prioritization, there isn't any by default but we do
have a prototype request scheduler that uses dmclock to prioritize
requests based on some hard-coded request classes. It's not especially
useful in its current form, but we do have plans to further elaborate
the classes and eventually pass the information down to osds for
integrated QOS.
As of nautilus, though, the thread pool size is the only effective knob
you have.
I'm also happy to look into the code to suggest improvements if you
can give me some quick points into the code to start will help.
----------------
Robert LeBlanc
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