Make --lru_mb optional

[campoy]

Currently, alphas won't work unless you provide a value for --lru_mb of at least 1024MB

We should instead set the default to 1024MB.

[MichelDiz]

Just an opinion, I think we should try catch the RAM available and set 50% of it as default.

e.g (from stackoverflow)

package main

// #include <unistd.h>
import "C"

func main() {
    println(C.sysconf(C._SC_PHYS_PAGES)*C.sysconf(C._SC_PAGE_SIZE), " bytes")
}

Or https://godoc.org/github.com/capnm/sysinfo

If we set 1024MB as default users can think this is some kind of minimal recommendation. But the real recommendation is to use at least 50% of available RAM.

[mangalaman93]

I do not think lru_mb is currently used at all. It was previously used when we had an LRU cache, it was removed from Dgraph at some point.

[campoy]

I don't care that much about the value, but making it compulsory when we're not even using it seems unnecessary.

Running the version on master as today I get:

➜  dgraph git:(master) ~/bin/dgraph alpha
[Decoder]: Using assembly version of decoder
2019/09/03 09:30:05 LRU memory (--lru_mb) must be specified. (At least 1024 MB)

[campoy]

After a chat with @manishrjain we decided that we should try to guess the total size of RAM and:

• if we find the size, use 50% of it and log a warning with a link to a page explaining how to set the lru_mb correctly
• if we don't find the size (as it probably happens with Windows?) fail and log an error with the same link as above

This will be part of the next release, not necessary for 1.1.0