Date: Thu, 25 Jul 2024 18:11:00 -0500 From: Jake Freeland <jake@technologyfriends.net> To: Mark Johnston <markj@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-hackers@freebsd.org Subject: Re: FreeBSD hugepages Message-ID: <912f849b-95ac-4d29-8a86-300999a0a9c4@technologyfriends.net> In-Reply-To: <ZqLUX47CAUAlq7nq@nuc> References: <1ced4290-4a31-4218-8611-63a44c307e87@technologyfriends.net> <ZqKhP0aR0fb_f6XE@kib.kiev.ua> <35da66f9-b913-45ea-90f4-16a2fa072848@technologyfriends.net> <ZqKzCK4pHg1mrSOa@nuc> <4d4398e5-81ba-4fbd-9806-649ec70abdb4@technologyfriends.net> <ZqLTA_tSP9dEQwil@nuc> <ZqLUX47CAUAlq7nq@nuc>
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On 7/25/24 17:40, Mark Johnston wrote: > On Thu, Jul 25, 2024 at 06:34:43PM -0400, Mark Johnston wrote: >> On Thu, Jul 25, 2024 at 04:11:22PM -0500, Jake Freeland wrote: >>> On 7/25/24 15:18, Mark Johnston wrote: >>>> On Thu, Jul 25, 2024 at 02:47:16PM -0500, Jake Freeland wrote: >>>>> On 7/25/24 14:02, Konstantin Belousov wrote: >>>>>> On Thu, Jul 25, 2024 at 01:46:17PM -0500, Jake Freeland wrote: >>>>>>> Hi there, >>>>>>> >>>>>>> I have been steadily working on bringing Data Plane Development Kit (DPDK) >>>>>>> on FreeBSD up to date with the Linux version. The most significant hurdle so >>>>>>> far has been supporting concurrent DPDK processes, each with their own >>>>>>> contiguous memory regions. >>>>>>> >>>>>>> These contiguous regions are used by DPDK as a heap for allocating DMA >>>>>>> buffers and other miscellaneous resources. Retrieving the underlying memory >>>>>>> and mapping these regions is currently different on Linux and FreeBSD: >>>>>>> >>>>>>> On Linux, hugepages are fetched from the kernel's pre-allocated hugepage >>>>>>> pool and are mapped into virtual address space on DPDK initialization. Since >>>>>>> the hugepages exist in a pool, multiple processes can reserve their own >>>>>>> hugepages and operate concurrently. >>>>>>> >>>>>>> On FreeBSD, DPDK uses an in-house contigmem kernel module that reserves a >>>>>>> large contiguous region of memory on load. During DPDK initialization, the >>>>>>> entire region is mapped into virtual address space. This leaves no memory >>>>>>> for another independent DPDK process, so only one process can operate at a >>>>>>> time. >>>>>>> >>>>>>> I could modify the DPDK contigmem module to mimic Linux's hugepages, but I >>>>>>> thought it would be better to integrate and upstream a hugepage-like >>>>>>> interface directly in the FreeBSD kernel source. I am writing this email to >>>>>>> see if anyone has any advice on the matter. I did not see any previous >>>>>>> attempts at this in Phabriactor or the commit log, but it is possible that I >>>>>>> missed it. I have read about transparent superpage promotion, but that seems >>>>>>> like a different mechanism altogether. >>>>>>> >>>>>>> At a quick glance, the implementation seems straightforward: read some >>>>>>> loader tunables, allocate persistent hugepages at boot time, and create a >>>>>>> pseudo filesystem that supports creating and mapping hugepages. I could be >>>>>>> underestimating the magnitude of this task, but that is why I'm asking for >>>>>>> thoughts and advice :) >>>>>>> >>>>>>> For reference, here is Linux's documentation on hugepages: >>>>>>> https://docs.kernel.org/admin-guide/mm/hugetlbpage.html >>>>>> Are posix shm largepages objects enough (they were developed to support >>>>>> DPDK). Look for shm_create_largepage(3). >>>>> Yes, shm_create_largepage(2) looks promising, but I would like the ability >>>>> to allocate these largepages at boot time when memory fragmentation as at a >>>>> minimum. Perhaps a couple sysctl tunables could be added onto the >>>>> vm.largepages node to specify a pagesize and allocate some number of pages >>>>> at boot? >>>> We could add an rc script which creates named largepage objects. This >>>> can be done using the posixshmcontrol utility. That might not be early >>>> enough during boot for some purposes. In that case, we could have a >>>> module which creates such objects from within the kernel. This is >>>> pretty straightforward to do; I wrote a dumb version of this for a >>>> mips-specific project a few years ago, feel free to take code or >>>> inspiration from it: https://people.freebsd.org/~markj/tlbdemo.c >>> Looks simple enough. Thanks for the example code. >>> >>>>> It seems Linux had an interface similar to shm_create_largepage(2) back in >>>>> v2.5, but they removed it in favor of their hugetlbfs filesystem. It would >>>>> be nice to stay close to the file-backed Linux interface to maximize code >>>>> sharing in userspace. It looks like the foundation for hugepages is there, >>>>> but the interface for allocation and access needs to be extended. >>>> POSIX shm objects have most of the properties one would want, I'd >>>> expect, save the ability to access them via standard syscalls. What >>>> else is missing besides the ability to reserve memory at boot time? >>> Most notably, I would like the ability to allocate pages in a specific NUMA >>> domain. >> I thought this was already supported, but it seems not... > Thinking a bit more, I'm pretty sure I had just been using something > like > > $ cpuset -n prefer:<domain> posixshmcontrol create -l 1G /largepage-1G-<domain> > > so didn't need an explicit NUMA configuration parameter. In C one would > use cpuset_setdomain(2) instead, but that's not as convenient. So, > imbuing a NUMA domain in struct shm_largepage_conf is still probably a > reasonable thing to do. I just looked at the code, this seems very manageable. I'll draft up a review. >> It should be very easy to implement: extend shm_largepage_conf to >> include a NUMA domain parameter, and specify that domain when allocating >> pages for the object (in shm_largepage_dotruncate(), the >> vm_page_alloc_contig() call should become a >> vm_page_alloc_contig_domain() call). >> >>> Otherwise, in a perfect world, I'd like a unified interface for both >>> Linux and FreeBSD. Linux hugepages are managed using standard system calls; >>> files are mmap(2)'d into virtual address space from hugetlbfs and >>> ftruncate(2)'d. >> largepage shm objects work this way as well. After reading through the man page, this is quite apparent. Not sure how I failed make that connection. Anyway, this is starting to look easier than I thought it would be. The only difference from a userspace perspective that I can think of right now is how the pages are created (e.g. hugetlbfs open(2) on Linux vs. shm_create_largepage(2) on FreeBSD). Thanks for the guidance Mark and Konstantin. Jake Freeland >>> A matching interface would not add an extra kernel >>> entrypoint and even more importantly, it would ease the Linux-to-FreeBSD >>> porting process for programs that use hugepages.
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