You really really really really really *must* not put data in memcached
which you can't lose.
Seriously, really don't do it. If you need persistence, try using a redis
instance for the persistent stuff, and use memcached for your cache stuff.
I don't see why you feel like you need to write your own thing, there're a
lot of persistent key/value stores (kyotocabinet/etc?). They have a much
lower request ceiling and don't handle the LRU/cache pattern as well, but
that's why you can use both.
Again, please please don't do it. You are damaging your company. You are a
*danger* to your company.
On Thu, 10 Apr 2014, Slawomir Pryczek wrote:
> Hi Dormando, thanks for suggestions, background thread would be nice...
> The idea is actually that with 2-3GB i get plenty of evictions of items that
> need to be fetched later. And with 16GB i still get evictions,
> actually probably i could throw more memory than 16G and it'd only result in
> more expired items sitting in the middle of slabs, forever... Now im
> going for persistence. Sounds probably crazy, but we're having some data that
> we can't loose:
> 1. statistics, we aggregate writes to DB using memcached (+list
> implementation). If these items get evicted we're loosing rows in db. Loosing
> data
> sometimes isn't a big problem. Eg. we restart memcached once a week so we're
> loosing 1 minute of data every week. But if we have evictions we're
> loosing data constantly (which we can't have)
> 2. we drive load balancer using data in memcached for statistics, again, not
> nice to loose data often because workers can get incorrect amount of
> traffic.
> 3. we're doing some adserving optimizations, eg. counting per-domain ad
> priority, for one domain it takes about 10 seconds to analyze all data and
> create list of ads, so can't be done online... we put result of this in
> memcached, if we loose too much of this the system will start to serve
> suboptimal ads (because it'll need to switch to more general data or much
> simpler algorithm that can be done instantly)
>
> Probably would be best to rewrite all this using C or golang, and use
> memcached just for caching, but it'd take too much time which we don't have
> currently...
>
> I have seen twitter and nk implementations that seem to do what i need, but
> they seem old (based on old code), so I prefer to modify code of recent
> "official" memcached, to not be stuck with old code or abandonware. Actually
> there are many topics about limitations of currrent eviction algo and
> option to enable some background thread to do scraping based on statistics of
> most filled slabs (with some parameter to specify if it should take
> light or aggressive approach) would be nice...
>
> As for the code... is that slab_rebalance_move function in slab.c? It seems a
> little difficult to gasp without some DOCs of how things are
> working... can you please write a very short description of how this "angry
> birds" more workd?
Look at doc/protocol.txt for explanations of the slab move options. the
names are greppable back to the source.
> I have quick question about this above... linked is item that's placed on
> linked list, but what other flags means, and why 2 last are 2 of them
> temporary?
> #define ITEM_LINKED 1
> #define ITEM_CAS 2
>
> /* temp */
> #define ITEM_SLABBED 4
> #define ITEM_FETCHED 8
>
> This from slab_rebalance_move seems interesting:
> refcount = refcount_incr(&it->refcount);
> ...
> if (refcount == 1) { /* item is unlinked, unused */
> ...
> } else if (refcount == 2) { /* item is linked but not busy */
>
> Is there some docs about refcounts, locks and item states? Basically why item
> with refcount 2 is not busy? You're increasing refcount by 1 on
> select, then again when reading data? Can refcount ever be higher than 2 (3
> in above case), meaning 2 threads can access same item?
The comment on the same line is explaining exactly what it means.
Unfortunately it's a bit of a crap shoot. I think I wrote a threads
explanation somewhnere (some release notes, or in a file in there, I can't
quite remember offhand). Since scaling the thread code it got a lot more
complicated. You have to be extremely careful under what circumstances you
access items (you must hold an item lock + the refcount must be 2 if you
want to unlink it).
You'll just have to study it a bit, sorry. Grep around to see where the
flags are used.
> Thanks.
>
> W dniu czwartek, 10 kwietnia 2014 06:05:30 UTC+2 użytkownik Dormando napisał:
> > Hi Guys,
> > im running a specific case where i don't want (actually can't have)
> to have evicted items (evictions = 0 ideally)... now i have
> created some simple
> > algo that lock the cache, goes through linked list and evicts
> items... it makes some problems, like 10-20ms cache locks on some
> cases.
> >
> > Now im thinking about going through each slab memory (slabs keep a
> list of allocated memory regions) ... looking for items, if
> expired item is
> > found, evict it... this way i can go eg. 10k items or 1MB of memory
> at a time + pick slabs with high utilization and run this
> "additional" eviction
> > only on them... so it'll prevent allocating memory just because
> unneded data with short TTL is occupying HEAD of the list.
> >
> > With this linked list eviction im able to run on 2-3GB of memory...
> without it 16GB of memory is exhausted in 1-2h and then memcached
> starts to
> > kill "good" items (leaving expired ones wasting memory)...
> >
> > Any comments?
> > Thanks.
>
> you're going a bit against the base algorithm. if stuff is falling out
> of
> 16GB of memory without ever being utilized again, why is that critical?
> Sounds like you're optimizing the numbers instead of actually tuning
> anything useful.
>
> That said, you can probably just extend the slab rebalance code.
> There's a
> hook in there (which I called "Angry birds mode") that drives a slab
> rebalance when it'd otherwise run an eviction. That code already safely
> walks the slab page for unlocked memory and frees it; you could edit it
> slightly to check for expiration and then freelist it into the slab
> class
> instead.
>
> Since it's already a background thread you could further modify it to
> just
> wake up and walk pages for stuff to evict.
>
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