tag proposal

Roy Lyseng Roy.Lyseng at Sun.COM
Tue Oct 9 07:35:44 UTC 2007

dormando wrote:
>>     A singly linked list should be sufficient.  The structs could be 
>> reused, but the actual tags themselves do need to be allocated at some 
>> point.  I don't know how important that allocation is to avoid.
> (Duh, sorry. I should've just said linked list instead of vague 
> struct/this/that).
> Allocations aren't bad if they're not happening constantly. I guess if 
> someone's going to have one tag per item you'd end up doing a malloc per 
> store anyway, which is bad...
>>     I meant 8 bytes per item overhead in general.  The counter and the 
>> pointer.  Whether the pointer is a linked list or an array, the 
>> overhead is fixed for non-tag users.
> Duh, again.
>>> Tag support could probably be a config (not ./configure) option 
>>> though, and avoid that memory overhead.
>>     That feels messy, but it could be made to work.
> IMHO territory, but I like ./configure'ing new experimental features, 
> and config'uring stable ones. Makes more sense for people who want to 
> use packages, which ends up being a lot.
>>     When do we need locks?
>>     I admit I'm a bit weak when it comes to threading in C, but when 
>> threading, I'd think the global counter should be something that 
>> doesn't get cached and where the increment is atomic.
> Are increments atomic across SMP/multicore? It'd be hard to corrupt it, 
> but I don't know off the top of my head if it's safe to ignore locking 
> when you're just reading/writing to one set of 4-8bytes. I'll have to 
> read up (or hope someone who knows better about this use of memory 
> barriers chimes in. It's not something I deal with much outside of 
> tracking down kernel bugs on SMP systems).

Increments are generally not atomic. You have interfaces like 
atomic_inc_ulong() in Solaris 10 and in the Linux kernel (AFAIK).

>>     The hashtable itself can at worst have a lock per bucket.  There's 
>> an implementation of a lock-free hashtable in java that should be 
>> possible in C as well.  Perhaps we could just leave this optimization 
>> to those who need it since the overlap between people who really need 
>> MT and people who really wants tags seems to be quite small so far.  :)
> Hah. Biglock the hash table, let someone who needs it fix it? :) Dirty, 
> but I can't complain.

Looking into vertically scalable versions of memcached I can just say 
thank you ;)

> -Dormando

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