On Thu 23 Apr 2015 12:15:04 PM CEST, Stefan Hajnoczi wrote:
>> For a cache size of 128MB, the PSS is actually ~10MB larger without
>> the patch, which seems to come from posix_memalign().
>
> Do you mean RSS or are you using a tool that reports a "PSS" number
> that I don't know about?
>
> We should understand what is going on instead of moving the code
> around to hide/delay the problem.
Both RSS and PSS ("proportional set size", also reported by the kernel).
I'm not an expert in memory allocators, but I measured the overhead like
this:
An L2 cache of 128MB implies a refcount cache of 32MB, in total 160MB.
With a default cluster size of 64k, that's 2560 cache entries.
So I wrote a test case that allocates 2560 blocks of 64k each using
posix_memalign and mmap, and here's how their /proc/<pid>/smaps compare:
-Size: 165184 kB
-Rss: 10244 kB
-Pss: 10244 kB
+Size: 161856 kB
+Rss: 0 kB
+Pss: 0 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
-Private_Dirty: 10244 kB
-Referenced: 10244 kB
-Anonymous: 10244 kB
+Private_Dirty: 0 kB
+Referenced: 0 kB
+Anonymous: 0 kB
AnonHugePages: 0 kB
Swap: 0 kB
KernelPageSize: 4 kB
Those are the 10MB I saw. For the record I also tried with malloc() and
the results are similar to those of posix_memalign().
Berto
