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:
>=20
> On 04/12/2019 00:28, Peter Eriksson wrote:
>> =E2=80=A6 and X levels deeper it gives this (not all filesystems =
mounted - I rebooted fairly quickly):
>>=20
>> 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=3DUMA 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
>>=20
>>=20
>> Hmm=E2=80=A6 I wonder if it isn=E2=80=99t this code I =
/usr/src/sys/sys/queue.h:
>>=20
>>> #define SLIST_REMOVE(head, elm, type, field) do { =
\
>>> QMD_SAVELINK(oldnext, (elm)->field.sle_next); =
\
>>> if (SLIST_FIRST((head)) =3D=3D (elm)) { =
\
>>> SLIST_REMOVE_HEAD((head), field); =
\
>>> } =
\
>>> else { =
\
>>> QUEUE_TYPEOF(type) *curelm =3D SLIST_FIRST(head); =
\
>>> while (SLIST_NEXT(curelm, field) !=3D (elm)) =
\
>>> curelm =3D SLIST_NEXT(curelm, field); =
\
>>> SLIST_REMOVE_AFTER(curelm, field); =
\
>>> } =
\
>>> TRASHIT(*oldnext); =
\
>>> } while (0)
>>=20
>> Combined with this in /usr/src/sys/vm/uma_core.c: keg_drain():
>>=20
>>> finished:
>>> KEG_UNLOCK(keg);
>>>=20
>>> while ((slab =3D SLIST_FIRST(&freeslabs)) !=3D NULL) {
>>> SLIST_REMOVE(&freeslabs, slab, uma_slab, us_hlink);
>>> keg_free_slab(keg, slab, keg->uk_ipers);
>>> }
>>=20
>> (The keg_drain print above is the time for the while() loop).
>>=20
>> If I=E2=80=99m reading this right it looks like it=E2=80=99s =
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.
>=20
> 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.
>=20
> 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 !=3D trash_fini)
> Looks like there is a "ton" of memory reading done here.
>=20
> --=20
> Andriy Gapon
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