Re: [Merge] ~dirk.zimoch/epics-base:dbChannelForDBLinks into epics-base:7.0

[Ben Franksen via Core-talk <core-talk at aps.anl.gov>]

Am 26.03.20 um 16:17 schrieb Dirk Zimoch via Core-talk:
> It seems what really eats up time is db_create_read_log(), not
> running the empty list of filters. Skipping it gives 200-400 ns
> instead of 800-1200 (sometimes 120000) ns when only skipping the
> filters. Running or not running empty filter lists makes almost no
> difference.

I am not surprised. Running two empty for-loops amounts to testing the
loop condition twice. That should have a negligable impact.

> My guess is on the free list mutex. Maybe a new,
> lock-free implementation of the freeList would help.

That sounds plausible. I wasn't aware that allocating from a freeList
needs to take a mutex lock but of course it makes sense that it has to.
Still, before spending time on a new freeList implementation I would try
to verify this with a profiler.

This isn't directly related, but I have been wondering about some of the
uses of freeLists for filters, particularly the array filter. The global
freeList for the filter parameters is okay, but we also use a separate
freeList for each filter to store the actual array data. This doesn't
make much sense to me: the filter is invoked post-queue, so the theread
that invokes it is also the one that consumes the result. So we could as
well re-use the same space for all requests during the lifetime of the
filter.

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