I've noticed the same thing after an hour of operating on a 
fairly high load site, although not as busy as colnect.net 
above.

Basically it seems like once memory is released by the cache 
it goes into a blank area on the Pie Chart and a black area 
on the bottom stacked bar graph, although the values for 
memory free and memory used are still correct.

It's a bit confusing to say the least.  What is happening 
here? Is this really a bug or does the blank/black space 
represent some other memory state?

Samples:

http://nixbox.com/images/apc/1.png
http://nixbox.com/images/apc/2.png
http://nixbox.com/images/apc/3.png

When I cleared the Opcode cache early the next day, it shows 
the APC was restarted entirely:

http://nixbox.com/images/apc/4.png

However does not reset this timer when the User cache is 
cleared.

When I clear both the Opcode AND the User cache, it mostly 
resets the blank/black areas of the charts back to 
Free/green:

http://nixbox.com/images/apc/5.png

Leading me to believe the blank/black areas are freed User 
cache entries but somehow not sent back into the Free pool?  
Not sure.  We have have the user cache entries set to expire 
fairly aggressively at 3 hours expiry.

and here are the APC settings incase that is helpful:

http://nixbox.com/images/apc/settings.png

Is there a different method that should be used to free the 
User cache entries, perhaps?

We just installed the new 3.1.6 APC class so I we are not 
yet sure if this strange fragmentation/release issue is 
affecting long term performance.

I have this same issue on 3.1.8 with PHP 5.3.3-7+squeeze1 
(Debian).
My graph, however, is opposite of theirs.
My graph shows slices with incorrect size, resulting in the 
slices summing up to over 100% of allocation.

Image is here; 
http://img859.imageshack.us/img859/8412/voilacapture44.png

after creating the username and password field with the submit field, how do i 
connect it with the microsoft access database, so that i can save the usernames 
and passwords of the users on my website. and by the way, i used adobe dreamweaver 
cs6 to create the php login page. please reply asap.

I'm wondering if this is a bug or some kind of memory leak as well.

We're using PHP 5.4.35 and APC 3.1.13

I know this is old, but it's really the only thread I can find relating to this.

My graph looks quite similar to the OP.  It's very noticeable about 24 hours later.  I have about half the pie sliced into about 1/8th of it being "used" space and the other being blank areas.  Also, it shows that it is 22.85% fragmented.  Is that the issue maybe?

According to <https://bugs.php.net/69618>, APC support has been
discontinued in favor of OPcache, APCu, the session upload
progress API and WinCache. Therefore this issue won't get fixed.