Date: Sat, 20 Apr 2024 11:23:41 -0600 From: Alan Somers <asomers@freebsd.org> To: Mark Johnston <markj@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Stressing malloc(9) Message-ID: <CAOtMX2jk6%2BSvqMP7Cbmdk0KQCFZ34yWuir7n_8ewZYJF2MwPSg@mail.gmail.com> In-Reply-To: <ZiPaFw0q17RGE7cS@nuc> References: <CAOtMX2jeDHS15bGgzD89AOAd1SzS_=FikorkCdv9-eAxCZ2P5w@mail.gmail.com> <ZiPaFw0q17RGE7cS@nuc>
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On Sat, Apr 20, 2024 at 9:07=E2=80=AFAM Mark Johnston <markj@freebsd.org> w= rote: > > On Fri, Apr 19, 2024 at 04:23:51PM -0600, Alan Somers wrote: > > TLDR; > > How can I create a workload that causes malloc(9)'s performance to plum= met? > > > > Background: > > I recently witnessed a performance problem on a production server. > > Overall throughput dropped by over 30x. dtrace showed that 60% of the > > CPU time was dominated by lock_delay as called by three functions: > > printf (via ctl_worker_thread), g_eli_alloc_data, and > > g_eli_write_done. One thing those three have in common is that they > > all use malloc(9). Fixing the problem was as simple as telling CTL to > > stop printing so many warnings, by tuning > > kern.cam.ctl.time_io_secs=3D100000. > > > > But even with CTL quieted, dtrace still reports ~6% of the CPU cycles > > in lock_delay via g_eli_alloc_data. So I believe that malloc is > > limiting geli's performance. I would like to try replacing it with > > uma(9). > > What is the size of the allocations that g_eli_alloc_data() is doing? > malloc() is a pretty thin layer over UMA for allocations <=3D 64KB. > Larger allocations are handled by a different path (malloc_large()) > which goes directly to the kmem_* allocator functions. Those functions > are very expensive: they're serialized by global locks and need to > update the pmap (and perform TLB shootdowns when memory is freed). > They're not meant to be used at a high rate. In my benchmarks so far, 512B. In the real application the size is mostly between 4k and 16k, and it's always a multiple of 4k. But it's sometimes great enough to use malloc_large, and it's those malloc_large calls that account for the majority of the time spent in g_eli_alloc_data. lockstat shows that malloc_large, as called by g_elI_alloc_data, sometimes blocks for multiple ms. But oddly, if I change the parameters so that g_eli_alloc_data allocates 128kB, I still don't see malloc_large getting called. And both dtrace and vmstat show that malloc is mostly operating on 512B allocations. But dtrace does confirm that g_eli_alloc_data is being called with 128kB arguments. Maybe something is getting inlined? I don't understand how this is happening. I could probably figure out if I recompile with some extra SDT probes, though. > > My first guess would be that your production workload was hitting this > path, and your benchmarks are not. If you have stack traces or lock > names from DTrace, that would help validate this theory, in which case > using UMA to cache buffers would be a reasonable solution. Would that require creating an extra UMA zone for every possible geli allocation size above 64kB? > > > But on a non-production server, none of my benchmark workloads causes > > g_eli_alloc_data to break a sweat. I can't get its CPU consumption to > > rise higher than 0.5%. And that's using the smallest sector size and > > block size that I can. > > > > So my question is: does anybody have a program that can really stress > > malloc(9)? I'd like to run it in parallel with my geli benchmarks to > > see how much it interferes. > > > > -Alan > >
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