RFR(s): 8150460: (linux|bsd|aix)_close.c: file descriptor table may become large or may not work at all

[Thomas Stüfe]

Hi Dmitry,

On Tue, Mar 1, 2016 at 1:44 PM, Dmitry Samersoff <
dmitry.samersoff at oracle.com> wrote:

> Christoph,
>
> > Dmitry, I think you are referring to an outdated version of the
> > webrev, the current one is this:
>
> Yes. Sorry!
>
> You may consider a bit different approach to save memory:
>
> Allocate multiple baseTables for different ranges of fd's with
> plain array of 32 * (fdEntry_t*) for simple case.
>
> i.e. if (fd < 32)
>          do plain array lookup
>
>      if (fd < N1)
>          do two steps lookup in baseTable1
>
>      if (fd < N2)
>          do two steps lookup in baseTable2
>
>
How does this differ from my approach in
http://cr.openjdk.java.net/~stuefe/webrevs/8150460-linux_close-fdTable/webrev.01/webrev/
 ?

For fd < 65535, I effectively fall back to a plain array lookup by setting
the size of the base table to 1. So, for this case the sparse array
degenerates to a one-dimensional plain array.

Kind Regards, Thomas



>      ...
>
> -Dmitry
>
>
>
> On 2016-03-01 13:47, Langer, Christoph wrote:
> > Hi Dmitry, Thomas,
> >
> > Dmitry, I think you are referring to an outdated version of the
> > webrev, the current one is this:
> >
> http://cr.openjdk.java.net/~stuefe/webrevs/8150460-linux_close-fdTable/webrev.01/webrev/
> >
> >  However, I agree - the lock should probably not be taken every time
> > but only in the case where we find the entry table was not yet
> > allocated.
> >
> > So, maybe getFdEntry should always do this: entryTable =
> > fdTable[rootArrayIndex]; // no matter if rootArrayIndex is 0
> >
> > Then check if entryTable is NULL and if yes then enter a guarded
> > section which does the allocation and before that checks if another
> > thread did it already.
> >
> > Also I'm wondering if the entryArrayMask and the rootArrayMask should
> > be calculated once in the init() function and stored in a static
> > field? Because right now it is calculated every time getFdEntry() is
> > called and I don't think this would be optimized by inlining...
> >
> > Best regards Christoph
> >
> > -----Original Message----- From: core-libs-dev
> > [mailto:core-libs-dev-bounces at openjdk.java.net] On Behalf Of Dmitry
> > Samersoff Sent: Dienstag, 1. März 2016 11:20 To: Thomas Stüfe
> > <thomas.stuefe at gmail.com>; Java Core Libs
> > <core-libs-dev at openjdk.java.net> Subject: Re: RFR(s): 8150460:
> > (linux|bsd|aix)_close.c: file descriptor table may become large or
> > may not work at all
> >
> > Thomas,
> >
> > Sorry for being later.
> >
> > I'm not sure we should take a lock at ll. 131 for each fdTable
> > lookup.
> >
> > As soon as we never deallocate fdTable[base_index] it's safe to try
> > to return value first and then take a slow path (take a lock and
> > check fdTable[base_index] again)
> >
> > -Dmitry
> >
> >
> > On 2016-02-24 20:30, Thomas Stüfe wrote:
> >> Hi all,
> >>
> >> please take a look at this proposed fix.
> >>
> >> The bug: https://bugs.openjdk.java.net/browse/JDK-8150460 The
> >> Webrev:
> >>
> http://cr.openjdk.java.net/~stuefe/webrevs/8150460-linux_close-fdTable/webrev.00/webrev/
> >>
> >>
> >>
> Basically, the file descriptor table implemented in linux_close.c may not
> >> work for RLIMIT_NO_FILE=infinite or may grow very large (I saw a
> >> 50MB table) for high values for RLIMIT_NO_FILE. Please see details
> >> in the bug description.
> >>
> >> The proposed solution is to implement the file descriptor table not
> >> as plain array, but as a twodimensional sparse array, which grows
> >> on demand. This keeps the memory footprint small and fixes the
> >> corner cases described in the bug description.
> >>
> >> Please note that the implemented solution is kept simple, at the
> >> cost of somewhat higher (some kb) memory footprint for low values
> >> of RLIMIT_NO_FILE. This can be optimized, if we even think it is
> >> worth the trouble.
> >>
> >> Please also note that the proposed implementation now uses a mutex
> >> lock for every call to getFdEntry() - I do not think this matters,
> >> as this is all in preparation for an IO system call, which are
> >> usually way more expensive than a pthread mutex. But again, this
> >> could be optimized.
> >>
> >> This is an implementation proposal for Linux; the same code found
> >> its way to BSD and AIX. Should you approve of this fix, I will
> >> modify those files too.
> >>
> >> Thank you and Kind Regards, Thomas
> >>
> >
> >
>
>
> --
> Dmitry Samersoff
> Oracle Java development team, Saint Petersburg, Russia
> * I would love to change the world, but they won't give me the sources.
>