Date: Wed, 27 Sep 2017 10:07:12 +0000 From: bugzilla-noreply@freebsd.org To: freebsd-bugs@FreeBSD.org Subject: [Bug 222639] fork: Time it takes to allocate random PID approach infinity as num_processes approach PID_MAX (related to: sysctl kern.randompid) Message-ID: <bug-222639-8@https.bugs.freebsd.org/bugzilla/>
next in thread | raw e-mail | index | archive | help
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D222639 Bug ID: 222639 Summary: fork: Time it takes to allocate random PID approach infinity as num_processes approach PID_MAX (related to: sysctl kern.randompid) Product: Base System Version: CURRENT Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: kern Assignee: freebsd-bugs@FreeBSD.org Reporter: marieheleneka@gmail.com The current implementation of random PID generation is a bit opportunistic. It currently generates a random value which is lastpid + (random_number cap= ped to a max value of kern.randompid), but always at least 1 more than lastpid. It then checks if this PID is available, and if not, tries again. Therefore, as number of processes approach PID_MAX (i.e. number of available PIDs approach 0), the time to allocate a randomly generated PID approaches infinity. Proposed solution: Prequisite: Change how PIDs are tracked and allocated. Implement a bitmap of PIDs (I'm currently working on this) which is updated whenever anything in = the PID namespace changes (a new PID/session ID/etc is created, or a PID/session ID/etc is considered available again).=20 The bitmap should be an array of 4096 uint32_t where each bit represents a = PID. A set (on) bit indicates an available PID. New random PID generation algorithm: Create an index of the bitmap consisting of 128 uint32_t. Each bit represen= ts a single uint32_t in the bitmap, declaring whether it has any available PIDs. When creating a random PID: - Iterate the index and find all uint32_t which have at least one bit set. - Select one of thee aforementioned at random. - Select a random set bit from the selected, to decide which uint32_t in the bitmap to use for further selection. - Select a random bit in the aforementioned to determine which PID to alloc= ate. Restrict allocation to lastpid < newpid < PID_MAX I'm working on this, but describing it here so it's not lost. For feedback = or just in case the "Bus Test" actually happens. ;) --=20 You are receiving this mail because: You are the assignee for the bug.=
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?bug-222639-8>