Date: Mon, 15 Oct 2012 23:57:46 +0200 From: Ivan Voras <ivoras@freebsd.org> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: freebsd-hackers@freebsd.org Subject: Re: NFS server bottlenecks Message-ID: <CAF-QHFXHq0%2BLd-Diu0jVhztwz68m%2BoH_AhzxNQF8S3QjQJ8uVw@mail.gmail.com> In-Reply-To: <1516511249.2287339.1350334696127.JavaMail.root@erie.cs.uoguelph.ca> References: <k5gtdh$nc0$1@ger.gmane.org> <1516511249.2287339.1350334696127.JavaMail.root@erie.cs.uoguelph.ca>
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On 15 October 2012 22:58, Rick Macklem <rmacklem@uoguelph.ca> wrote: > The problem is that UDP entries very seldom time out (unless the > NFS server isn't seeing hardly any load) and are mostly trimmed > because the size exceeds the highwater mark. > > With your code, it will clear out all of the entries in the first > hash buckets that aren't currently busy, until the total count > drops below the high water mark. (If you monitor a busy server > with "nfsstat -e -s", you'll see the cache never goes below the > high water mark, which is 500 by default.) This would delete > entries of fairly recent requests. You are right about that, if testing by Nikolay goes reasonably well, I'll work on that. > If you are going to replace the global LRU list with ones for > each hash bucket, then you'll have to compare the time stamps > on the least recently used entries of all the hash buckets and > then delete those. If you keep the timestamp of the least recent > one for that hash bucket in the hash bucket head, you could at least > use that to select which bucket to delete from next, but you'll still > need to: > - lock that hash bucket > - delete a few entries from that bucket's lru list > - unlock hash bucket > - repeat for various buckets until the count is beloew the high > water mark Ah, I think I get it: is the reliance on the high watermark as a criteria for cache expiry the reason the list is a LRU instead of an ordinary unordered list? > Or something like that. I think you'll find it a lot more work that > one LRU list and one mutex. Remember that mutex isn't held for long. It could be, but the current state of my code is just groundwork for the next things I have in plan: 1) Move the expiry code (the trim function) into a separate thread, run periodically (or as a callout, I'll need to talk with someone about which one is cheaper) 2) Replace the mutex with a rwlock. The only thing which is preventing me from doing this right away is the LRU list, since each read access modifies it (and requires a write lock). This is why I was asking you if we can do away with the LRU algorithm. > Btw, the code looks very nice. (If I was being a style(9) zealot, > I'd remind you that it likes "return (X);" and not "return X;". Thanks, I'll make it more style(9) compliant as I go along.
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