On Mon, Jun 22, 2009 at 9:29 AM, Mike Beltzner <[email protected]> wrote:
> On 21-Jun-09, at 10:22 AM, Mike Belshe wrote:
>
> Second, the author is basically right. Since he's running on Vista, its a
>> bit hard to tell whether his stats included shared memory or not; using the
>> default memory statistic ("Memory (Private Working Set)") is actually a
>> pretty good measure to just sum. But he doesn't say which measurement he
>> used.
>>
>
> Doesn't Private Bytes accurately represent the private memory for a given
> process? I thought the whole point of that was that it didn't measure any
> shared memory pools.
Yes, that accurately represents the private memory for a process, but it
doesn't reflect the user's experience. Windows generally tracks working
set. Why? Because the working set is the amount of memory *not available
to other apps*. If other apps can have the memory, then using the bytes is
inconsequential.
For most applications, there isn't much difference between private bytes and
working set private bytes. However, because of Chrome's multi-proc
architecture, there is a big difference. The reason is because
Chrome intentionally gives memory back to the OS. For instance, on my
current instance of Chrome, I'm using 16 tabs. The sum of the private bytes
is 514408. The sum of the private working set bytes is 275040, nearly half
of the private bytes number. This is on a machine with 8GB of RAM, so there
is plenty of memory to go around. But if some other app wants the memory,
Chrome gave it back to the OS and will suffer the page faults to get it
back. Since Chrome has given it back to the OS (and has volunteered to take
the performance consequences of getting it back), I don't think it should be
counted as Chrome usage. Others may disagree. But Windows uses working set
as the primary metric for all applications the OS folks agree that working
set is the right way to account for memory usage.
Single process browsers have a hard time giving memory back, because they
can't differentiate which pages are accounted to unused, background tabs and
which pages are accounted to the active, in-use tabs.
Finally, the common metric which definitely doesn't work well is Windows
XP's default metric: working set (private + shared). Because of shared
memory between processes, simply summing the working set will far overstate
the actual RAM used. Part of the motivation with Vista changing the default
metric from working set to private working set was precisely to deal with
the issue of better accounting of shared memory.
Mike
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