On Wed, Dec 11, 2019 at 5:29 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> I'd very much argue that it IS a bug, maybe just doesn't show on your
> system.
Oh, I agree. But I also understand why people hadn't noticed, and I
don't think it's all that critical - because if you do 1M iops cached,
you're doing something really really strange.
I too can see xas_create using 30% CPU time, but that's when I do a
perf record on just kswapd - and when I actually look at it on a
system level, it looks nowhere near that bad.
So I think people should look at this. Part of it might be for Willy:
does that xas_create() need to be that expensive? I hate how "perf"
callchains work, but it does look like it is probably
page_cache_delete -> xas_store -> xas_create that is the bulk of the
cost there.
Replacing the real page with the shadow entry shouldn't be that big of
a deal, I would really hope.
Willy, that used to be a __radix_tree_lookup -> __radix_tree_replace
thing, is there perhaps some simple optmization that could be done on
the XArray case here?
Linus
