Key Size

[Steven Grimm]

Philip Neustrom wrote:
> Steve mentions that with tens of millions of items in a cache, that
> the cost of having an additional byte is too great.  If I'm not
> mistaken, this is only 10MB of extra memory used, and in this
> situation (10 million items), you're looking at a cache which is
> likely in the 100's of GBs.  This doesn't seem like a measurable
> difference to me (less than .01 percent?)

That assumes all your objects are large. We store a lot of very short 
values in our cache, just a few bytes in many cases. On some of our 
memcached instances, we often see upwards of 40 million entries (that's 
per instance, not total across all instances.) You're correct that 
that's less than a percent of the cache, but it's closer to .5% than 
.01%, easily in the "measurable" range. I can say that with certainty 
since we've actually measured the difference as we've made various 
changes to the item structure over time.

If someone changes memcached to use a 2-byte key length, I suppose I 
won't object too much -- .5% is significant but not deadly -- but we 
certainly have zero use for such a change here. Just make sure you do it 
in such a way that byte alignment requirements don't cause you to add 
padding to the item structure, especially on 64-bit systems; .5% is not 
too bad but 4% (worst case, if you end up adding 8 bytes per item) 
certainly would be. And of course obviously Brad gets the final say!

-Steve