On Thu, Feb 3, 2022 at 7:13 PM Dmitry Kozlyuk <dkozl...@nvidia.com> wrote: > > This patchset is a new design and implementation of [1]. > > # Problem Statement > > Large allocations that involve mapping new hugepages are slow. > This is problematic, for example, in the following use case. > A single-process application allocates ~1TB of mempools at startup. > Sometimes the app needs to restart as quick as possible. > Allocating the hugepages anew takes as long as 15 seconds, > while the new process could just pick up all the memory > left by the old one (reinitializing the contents as needed). > > Almost all of mmap(2) time spent in the kernel > is clearing the memory, i.e. filling it with zeros. > This is done if a file in hugetlbfs is mapped > for the first time system-wide, i.e. a hugepage is committed > to prevent data leaks from the previous users of the same hugepage. > For example, mapping 32 GB from a new file may take 2.16 seconds, > while mapping the same pages again takes only 0.3 ms. > Security put aside, e.g. when the environment is controlled, > this effort is wasted for the memory intended for DMA, > because its content will be overwritten anyway. > > Linux EAL explicitly removes hugetlbfs files at initialization > and before mapping to force the kernel clear the memory. > This allows the memory allocator to clean memory on only on freeing. > > # Solution > > Add a new mode allowing EAL to remap existing hugepage files. > While it is intended to make restarts faster in the first place, > it makes any startup faster except the cold one > (with no existing files). > > It is the administrator who accepts security risks > implied by reusing hugepages. > The new mode is an opt-in and a warning is logged. > > The feature is Linux-only as it is related > to mapping hugepages from files which only Linux does. > It is inherently incompatible with --in-memory, > for --huge-unlink see below. > > There is formally no breakage of API contract, > but there is a behavior change in the new mode: > rte_malloc*() and rte_memzone_reserve*() may return dirty memory > (previously they were returning clean memory from free heap elements). > Their contract has always explicitly allowed this, > but still there may be users relying on the traditional behavior. > Such users will need to fix their code to use the new mode. > > # Implementation > > ## User Interface > > There is --huge-unlink switch in the same area to remove hugepage files > before mapping them. It is infeasible to use with the new mode, > because the point is to keep hugepage files for fast future restarts. > Extend --huge-unlink option to represent only valid combinations: > > * --huge-unlink=existing OR no option (for compatibility): > unlink files at initialization > and before opening them as a precaution. > > * --huge-unlink=always OR just --huge-unlink (for compatibility): > same as above + unlink created files before mapping. > > * --huge-unlink=never: > the new mode, do not unlink hugepages files, reuse them. > > This option was always Linux-only, but it is kept as common > in case there are users who expect it to be a no-op on other systems. > (Adding a separate --huge-reuse option was also considered, > but there is no obvious benefit and more combinations to test.) > > ## EAL > > If a memseg is mapped dirty, it is marked with RTE_MEMSEG_FLAG_DIRTY > so that the memory allocator may clear the memory if need be. > See patch 5/6 description for details how this is done > in different memory mapping modes. > > The memory manager tracks whether an element is clean or dirty. > If rte_zmalloc*() allocates from a dirty element, > the memory is cleared before handling it to the user. > On freeing, the allocator joins adjacent free elements, > but in the new mode it may not be feasible to clear the free memory > if the joint element is dirty (contains dirty parts). > In any case, memory will be cleared only once, > either on freeing or on allocation. > See patch 3/6 for details. > Patch 2/6 adds a benchmark to see how time is distributed > between allocation and freeing in different modes. > > Besides clearing memory, each mmap() call takes some time. > For example, 1024 calls for 1 TB may take ~300 ms. > The time of one call mapping N hugepages is O(N), > because inside the kernel hugepages are allocated ony by one. > Syscall overhead is negligeable even for one page. > Hence, it does not make sense to reduce the number of mmap() calls, > which would essentially move the loop over pages into the kernel. > > [1]: http://inbox.dpdk.org/dev/20211011085644.2716490-3-dkozl...@nvidia.com/ >
I fixed some checkpatch warnings, updated MAINTAINERS for the added test and kept ERR level for a log message when creating files in get_seg_fd(). Thanks again for enhancing the documentation, Dmitry. And thanks to testers and reviewers. Series applied. -- David Marchand
