Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: do lazy allocation of the L2 cache

[Alberto Garcia]

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