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Re: The Hurd: what is it?


From: Sergio Lopez
Subject: Re: The Hurd: what is it?
Date: Wed, 09 Nov 2005 23:22:33 +0100

El mié, 09-11-2005 a las 20:32 +0100, Marcus Brinkmann escribió:
> At Wed, 9 Nov 2005 19:07:50 +0100,
> Sergio Lopez <address@hidden> wrote:
> > On the other hand, I don't think that the IBM case is applicable for us,
> > since their objetives are far different from ours.
> 
> You think so?  Surely, they had a strong priority on multiple
> personalities, while we only have one personality.  Supporting
> multiple personalities in the Hurd is advertised as one of the
> advantages, though, and Workplace raises questions about that claim.
> 
> However, note that there are a number of results in the paper which
> have nothing to do with multiple personalities, but everything with a
> multi-server design on top of Mach.  Take for example the performance
> numbers for the file-server intensive tasks.  Workplace OS/2 ran 3
> times slower than OS/2.  And this is _after_ they made IPC synchronous
> and improved its performance 6-10 fold.
> 

We don't know (since the papers are not so technical, and we can't look
into the code) in deep what they do to Mach or how they implemented the
servers. We just know that they changed Mach in many ways (IPC
synchronous in a unknown way, user-space device drivers...) and that
they implemented their servers with other prorities in mind than ours.

And if Workplace works about 3 times slower than OS/2, they must did
something wrong, since that's more or less the rate of slowness for Hurd
vs. Linux with an unoptimized, asynchronous IPC ;-)

> The Liedtke papers shed a light on why this is so.  The problem is not
> purely the IPC performance.  A big issue is cache consumption: If the
> kernel's working set is bigger than the cache, there is a strange
> effect: Performance degrades as you increase the cache size!  This is
> because the kernel is eating the cache, and it needs to be refilled.
> This is why microkernels must be small, as in actual number of bytes.
> 

Having a small microkernel in number of bytes is just one way to
archieve a small working set, but not the only one. Probably will be
interesting to research this kind of this fine-grained optimization at
some point, but I don't see it as a priority and I don't think that this
is the main reason for Mach IPC slowness.

> > > For problems with the Hurd passive translator design:
> > > 
> > > http://lists.gnu.org/archive/html/l4-hurd/2005-10/msg00081.html
> > > 
> > > Active translators also must be considered harmful:
> > > 
> > > http://lists.gnu.org/archive/html/hurd-devel/2003-10/msg00002.html
> > > 
> > > (There is not a complete explanation of all the possible problems, but
> > > there are many examples, please use this as an opportunity for an
> > > exercise---check out how Linux FUSE does it).
> > > 
> > 
> > This is a interesting thing to discuss about. Do you really think that this
> > can't be solved from our current implementation?
> 
> The passive translator problem may not be worth paying any attention
> to, _unless_ you also want to fix a number of other issues, like
> resource accountability and flexible security.  But these do require
> larger architectural changes anyway.
> 
> The active translator problem seems serious to me.  Without any
> guarantee about the implementation of a service, you can not know what
> it does.  This means that you must be prepared for any malicious
> behaviour, including: no response (stalling the client), infinite
> virtual directory tree, confusing inode numbers and link counts,
> rapidly changing filesystem structure (to trigger race conditions) etc
> etc.
> 
> This is why in FUSE, users don't see the user filesystems of other
> users.  I am afraid that given the seriousness of the problem, this is
> the only sane option.  Only with a broader semantic framework can you
> re-enable sharing on a case by case basis.
> 
> Talking only about "safe" translators now: There is also the general
> question that _if_ you want to make all applications translator aware
> (good luck ;), which policy the programs should use with which
> translators?  Ie, which translators should they follow and which not?
> How do they even know what translator is running on a node?
> 
> The only thing that seems feasible at all is to have the translator
> advice the applications by means of a "follow me" stat bit.  This is a
> bit inflexible, but at least it calls for a consistent policy across
> all applications.
> 

Then this is something that we can solve by extending our current
implementation, isn't it? (maybe not with the world's best solution, but
with a working one :-)

> > > If you are looking at the actual Hurd implementation, you can find
> > > plenty of denial of service attack possibilities, as well as denial of
> > > resource attack possibilites.  No need to even try to enumerate them
> > > all.  Note the absence of a quota system.  Note the absence of
> > > feasibility to implement a quota system in such a multi-server system,
> > > without some ground-breaking architectural changes.
> > >
> > 
> > Mach knows about almost every resource allocation that the servers do, so I
> > don't think that will be extremly hard to solve this without completly 
> > breaking our current design.
> 
> Mach doesn't know on whose behalf the server does the allocation.
>   

Yes, but it knows about the server and the resource. We can take this as
a primitive.

> > > Just as an illustration: The number of worker threads in the system
> > > per server is unlimited.  It's not unusual for ext2fs to create 2000
> > > threads on page pressure, because Mach swaps them out individually.
> > > You can throttle, but not limit the number, because of the possibility
> > > for deadlocks.  There are many things wrong with that, starting from
> > > the simple fact that the number of threads in a server should be a
> > > function of the server design, and not of the number of users or
> > > system load.
> > 
> > This issue will be completly solved by implementing Migrating Threads on
> > GNU Mach, which is on the way. Other work, like partly reducing IPC
> > semantics or copying by temporary mapping can also be done to put the 
> > performance to a reasonable value.
> 
> Well, there is no deep architectural flaw in this example, so
> implementing migrating threads isn't even necessary to fix it.  It's
> just one of the many things that require attention.  You did ask for a
> _complete_ analysis, didn't you? :)
> 

The migrating thread implementation doesn't only solve the problem of
server worker threads, but also makes RPCs simpler, efficient and
(mostly) syncrhonous while retaining compatibility for asynchronous
operations. We know that this also introduces some new problems, but we
really think about it as a step forward.

> Everybody agrees that synchronous message delivery is important to get
> decent IPC performance.  It would be interesting to move the Hurd on
> Mach to a synchronous IPC design.  There are a couple of places where
> the Hurd relies on asynchronous delivery (and reply!!!), but those are
> rare.  I am not sure about Mach, though (external memory objects,
> notifications!?).  So, there may be some unexpected problems, and you
> might have to make a couple of compromises, but it may be feasible.
> 

Of course, at some point, IPC analysis and simplication will be needed.
And yes, it will require changes to some interfaces (mostly to the
external pagers interface, which IMHO will get a great benefit from
being synchronous, as this is its natural path for most operations), but
that's (by far) less work than redesigning/rewriting Hurd.

> I am not sure if you really want completely passive objects.  But I
> haven't thought about it much.  Maybe I am misunderstanding something.
> In the Hurd, objects are active.  Do you plan to change that?
> 

Migrating threads implies passive objects.

-- 
Sergio Lopez
address@hidden





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