On Thu, Apr 16, 2026 at 12:29:50PM +0200, Mateusz Guzik wrote: > On Tue, Apr 14, 2026 at 11:11 AM Huang Shijie <[email protected]> wrote: > > > > On Mon, Apr 13, 2026 at 05:33:21PM +0200, Mateusz Guzik wrote: > > > On Mon, Apr 13, 2026 at 02:20:39PM +0800, Huang Shijie wrote: > > > > In NUMA, there are maybe many NUMA nodes and many CPUs. > > > > For example, a Hygon's server has 12 NUMA nodes, and 384 CPUs. > > > > In the UnixBench tests, there is a test "execl" which tests > > > > the execve system call. > > > > > > > > When we test our server with "./Run -c 384 execl", > > > > the test result is not good enough. The i_mmap locks contended heavily > > > > on > > > > "libc.so" and "ld.so". For example, the i_mmap tree for "libc.so" can > > > > have > > > > over 6000 VMAs, all the VMAs can be in different NUMA mode. > > > > The insert/remove operations do not run quickly enough. > > > > > > > > patch 1 & patch 2 are try to hide the direct access of i_mmap. > > > > patch 3 splits the i_mmap into sibling trees, and we can get better > > > > performance with this patch set: > > > > we can get 77% performance improvement(10 times average) > > > > > > > > > > To my reading you kept the lock as-is and only distributed the protected > > > state. > > > > > > While I don't doubt the improvement, I'm confident should you take a > > > look at the profile you are going to find this still does not scale with > > > rwsem being one of the problems (there are other global locks, some of > > > which have experimental patches for). > > IMHO, when the number of VMAs in the i_mmap is very large, only optimise > > the rwsem > > lock does not help too much for our NUMA case. > > > > In our NUMA server, the remote access could be the major issue. > > > > I'm confused how this is not supposed to help. You moved your data to > be stored per-domain. With my proposal the lock itself will also get > that treatment. > > Modulo the issue of what to do with code wanting to iterate the entire > thing, this is blatantly faster. >
I tested an old lock patch yesterday. It really helps a lot. The lock patch is from this link: https://lkml.org/lkml/2024/9/14/280 The test results: v7.0-rc5 + (lock patch) : improve about %60% v7.0-rc5 + (lock patch) + (this patch set) : improve about 130% > > > > > > > > Apart from that this does nothing to help high core systems which are > > > all one node, which imo puts another question mark on this specific > > > proposal. > > Yes, this patch set only focus on the NUMA case. > > The one-node case should use the original i_mmap. > > > > Maybe I can add a new config, CONFIG_SPILT_I_MMAP. The config is disabled > > by default, and enabled when the NUMA node is not one. > > > > > > > > Of course one may question whether a RB tree is the right choice here, > > > it may be the lock-protected cost can go way down with merely a better > > > data structure. > > > > > > Regardless of that, for actual scalability, there will be no way around > > > decentralazing locking around this and partitioning per some core count > > > (not just by numa awareness). > > > > > > Decentralizing locking is definitely possible, but I have not looked > > > into specifics of how problematic it is. Best case scenario it will > > > merely with separate locks. Worst case scenario something needs a fully > > > stabilized state for traversal, in that case another rw lock can be > > Yes. > > > > The traversal may need to hold many locks. > > > > The very paragraph you partially quoted answers what to do in that > case: wrap everything with a new rwsem taken for reading when > adding/removing entries and taken for writing when iterating the > entire thing. Then the iteration sticks to one lock. > > The new rw lock puts an upper ceiling on scalability of the thing, but > it is way higher than the current state. Could you tell me the patch about it? Is this lock patch merged ? or not? I can test it. > > Given the extra overhead associated with it one could consider > sticking to one centralized state by default and switching to > distributed state if there is enough contention. > > > > slapped around this, creating locking order read lock -> per-subset > > > write lock -- this will suffer scalability due to the read locking, but > > > it will still scale drastically better as apart from that there will be > > > no serialization. In this setting the problematic consumer will write > > > lock the new thing to stabilize the state. > > > > > > So my non-maintainer opinion is that the patchset is not worth it as it > > > fails to address anything for significantly more common and already > > > affected setups. > > This patch set is to reduce the remote access latency for insert/remove VMA > > in NUMA. > > > > And I am saying the mmap semaphore is a significant problem already on > high-core no-numa setups. Addressing scalability in that case would > sort out the problem in your setup and to a significantly higher > extent. I am afraid even the lock patch resolves the scalability high-core no-numa setups, we still need to split the i_mmap for NUMA. > > > > > > > Have you looked into splitting the lock? > > > > > I ever tried. > > > > But there are two disadvantages: > > 1.) The traversal may need to hold many locks which makes the > > code very horrible. > > > > I already above this is avoidable. > > > 2.) Even we split the locks. Each lock protects a tree, when the tree > > becomes > > big enough, the VMA insert/remove will also become slow in NUMA. > > The reason is that the tree has VMAs in different NUMA nodes. > > > > This is orthogonal to my proposal. In fact, if one is to pretend this > is never a factor with your patch, I would like to point out it will > remain not a factor if the per-numa struct gets its own lock. Yes. It is orthogonal to your proposal. Thanks Huang Shijie
