Commit r345200 (new ARC reclamation threads) looks suspicious to me - second potential problem
ian at freebsd.org
Mon May 20 16:20:50 UTC 2019
On Mon, 2019-05-20 at 19:05 +0300, Lev Serebryakov wrote:
> I'm looking at last commit to
> 'sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c' (r345200) and
> have another question.
> Here are such code:
> 4960 /*
> 4961 * Kick off asynchronous kmem_reap()'s of all our
> 4962 */
> 4963 arc_kmem_reap_soon();
> 4965 /*
> 4966 * Wait at least arc_kmem_cache_reap_retry_ms between
> 4967 * arc_kmem_reap_soon() calls. Without this check it is
> possible to
> 4968 * end up in a situation where we spend lots of time
> 4969 * caches, while we're near arc_c_min. Waiting here
> gives the
> 4970 * subsequent free memory check a chance of finding
> that the
> 4971 * asynchronous reap has already freed enough memory,
> we don't
> 4972 * need to call arc_reduce_target_size().
> 4973 */
> 4974 delay((hz * arc_kmem_cache_reap_retry_ms + 999) /
> But looks like `arc_kmem_reap_soon()` is synchronous on FreeBSD! So,
> this `delay()` looks very wrong. Am I right?
> Looks like it should be `#ifdef illumos`.
One of the things arc_kmem_reap_soon() does is call
dnlc_reduce_cache(), and that sets a variable and does a condition
variable broadcast, presumably causing other threads to wake up and do
some work. So, presumably the delay (which appears to really be a call
to pause(9) on freebsd) allows time for that async work to happen
before calling arc_available_memory().
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