Date: Wed, 4 Dec 2019 11:13:43 +0100 From: Peter Eriksson <pen@lysator.liu.se> To: FreeBSD FS <freebsd-fs@freebsd.org> Cc: Andriy Gapon <avg@FreeBSD.org> Subject: Re: Slow reboots due to ZFS cleanup in kern_shutdown() .. zio_fini() Message-ID: <BDE8C3F8-7D55-4C4F-8E40-0DF2FD732D5A@lysator.liu.se> In-Reply-To: <e15d66e2-dde4-aaff-87fb-cacd15c1e719@FreeBSD.org> References: <AD17E454-6A51-436D-A853-07F04A406EC9@lysator.liu.se> <D2A11CE9-9B24-4E40-A51A-8D318E0288C9@lysator.liu.se> <20191202225424.GG43802@raichu> <3b71fe37-c29f-e3e5-ff96-5dce15cc7553@FreeBSD.org> <20191203162219.GI43802@raichu> <247C6990-BC6E-4E3D-8CEF-5A861D8A25EC@lysator.liu.se> <4D44B25E-2F15-4536-A653-DA242051C8A9@lysator.liu.se> <e15d66e2-dde4-aaff-87fb-cacd15c1e719@FreeBSD.org>
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Yeah, I noticed that after I sent that email (should send stuff just before you go to bed :-)
Anyway, some more timing/debug printfs point to the keg->uk_freef(mem) call in keg_free_slab() which sometimes take about a second to execute.
(Should probably switch to some other time source with greater resolution than 1s next :-)
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
keg_free_slab: keg->uk_freef(mem) took 1 seconds
- Peter
> On 4 Dec 2019, at 10:48, Andriy Gapon <avg@FreeBSD.org> wrote:
>
> On 04/12/2019 00:28, Peter Eriksson wrote:
>> … and X levels deeper it gives this (not all filesystems mounted - I rebooted fairly quickly):
>>
>> keg_drain: while (SLIST_FIRST/SLIST_REMOVE)-loop took 14 seconds [20021 loops]
>> zone_drain_wait(): zone_foreach_keg(zone, &keg_drain) took 14 seconds
>> zone_dtor(): zone_drain_wait(zone, M_WAITOK) took 14 seconds
>> zone_free_item(zone=UMA Zones): zone->uz_dtor() took 14 seconds
>> uma_zdestroy(zio_buf_12288) took 14 seconds
>> kmem_cache_destroy: uma_zdestroy(0xfffff803467c8ac0) [zio_buf_12288] took 14 seconds
>> kmem_cache_destroy(zio_buf_cache[20]) took 14 seconds
>>
>>
>> Hmm… I wonder if it isn’t this code I /usr/src/sys/sys/queue.h:
>>
>>> #define SLIST_REMOVE(head, elm, type, field) do { \
>>> QMD_SAVELINK(oldnext, (elm)->field.sle_next); \
>>> if (SLIST_FIRST((head)) == (elm)) { \
>>> SLIST_REMOVE_HEAD((head), field); \
>>> } \
>>> else { \
>>> QUEUE_TYPEOF(type) *curelm = SLIST_FIRST(head); \
>>> while (SLIST_NEXT(curelm, field) != (elm)) \
>>> curelm = SLIST_NEXT(curelm, field); \
>>> SLIST_REMOVE_AFTER(curelm, field); \
>>> } \
>>> TRASHIT(*oldnext); \
>>> } while (0)
>>
>> Combined with this in /usr/src/sys/vm/uma_core.c: keg_drain():
>>
>>> finished:
>>> KEG_UNLOCK(keg);
>>>
>>> while ((slab = SLIST_FIRST(&freeslabs)) != NULL) {
>>> SLIST_REMOVE(&freeslabs, slab, uma_slab, us_hlink);
>>> keg_free_slab(keg, slab, keg->uk_ipers);
>>> }
>>
>> (The keg_drain print above is the time for the while() loop).
>>
>> If I’m reading this right it looks like it’s potentially doing a linear search through a (long) linked list, for every entry in the free slabs list. 20000 entries in 14 seconds is 1400 entries per second.
>
> It's not a search. The loop is removing *all* items. There is no other way to
> do it :-)
> Expanding your calculations, it takes on the order of a millisecond to process a
> single slab. I bet that the time is spent in keg_free_slab() and it's a lot of
> time.
>
> I wonder if the OR should actually be AND in this condition:
> if (!uma_dbg_kskip(keg, slab->us_data + (keg->uk_rsize * i)) ||
> keg->uk_fini != trash_fini)
> Looks like there is a "ton" of memory reading done here.
>
> --
> Andriy Gapon
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