path: root/open_issues/performance.mdwn

[[!meta copyright="Copyright © 2010, 2011, 2012, 2013, 2014 Free Software
Foundation, Inc."]]

[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
id="license" text="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
is included in the section entitled [[GNU Free Documentation
License|/fdl]]."]]"""]]

*Performance analysis* ([[!wikipedia Performance_analysis desc="Wikipedia
article"]]) deals with analyzing how computing resources are used for
completing a specified task.

[[Profiling]] is one relevant tool.

In [[microkernel]]-based systems, there is generally a considerable [[RPC]]
overhead.

In a multi-server system, it is non-trivial to implement a high-performance
[[I/O System|community/gsoc/project_ideas/disk_io_performance]].

When providing [[faq/POSIX_compatibility]] (and similar interfaces) in an
environemnt that doesn't natively implement these interfaces, there may be a
severe performance degradation.  For example, in this [[`fork` system
call|/glibc/fork]]'s case.

[[Unit_testing]] can be used for tracking performance regressions.

---

  * [[Degradation]]

  * [[fork]]

  * [[IPC_virtual_copy]]

  * [[microbenchmarks]]

  * [[microkernel_multi-server]]

  * [[gnumach_page_cache_policy]]

  * [[metadata_caching]]

  * [[community/gsoc/project_ideas/object_lookups]]

---


# IRC, freenode, #hurd, 2012-07-05

    <braunr> the more i study the code, the more i think a lot of time is
      wasted on cpu, unlike the common belief of the lack of performance being
      only due to I/O


## IRC, freenode, #hurd, 2012-07-23

    <braunr> there are several kinds of scalability issues
    <braunr> iirc, i found some big locks in core libraries like libpager and
      libdiskfs
    <braunr> but anyway we can live with those
    <braunr> in the case i observed, ext2fs, relying on libdiskfs and libpager,
      scans the entire file list to ask for writebacks, as it can't know if the
      pages are dirty or not
    <braunr> the mistake here is moving part of the pageout policy out of the
      kernel
    <braunr> so it would require the kernel to handle periodic synces of the
      page cache
    <antrik> braunr: as for big locks: considering that we don't have any SMP
      so far, does it really matter?...
    <braunr> antrik: yes
    <braunr> we have multithreading
    <braunr> there is no reason to block many threads while if most of them
      could continue
    <braunr> -while
    <antrik> so that's more about latency than throughput?
    <braunr> considering sleeping/waking is expensive, it's also about
      throughput
    <braunr> currently, everything that deals with sleepable locks (both
      gnumach and the hurd) just wake every thread waiting for an event when
      the event occurs (there are a few exceptions, but not many)
    <antrik> ouch


## [[!message-id "20121202101508.GA30541@mail.sceen.net"]]


## IRC, freenode, #hurd, 2012-12-04

    <damo22> why do some people think hurd is slow? i find it works well even
      under heavy load inside a virtual machine
    <braunr> damo22: the virtual machine actually assists the hurd a lot :p
    <braunr> but even with that, the hurd is a slow system
    <damo22> i would have thought it would have the potential to be very fast,
      considering the model of the kernel
    <braunr> the design implies by definition more overhead, but the true cause
      is more than 15 years without optimization on the core components
    <braunr> how so ?
    <damo22> since there are less layers of code between the hardware bare
      metal and the application that users run
    <braunr> how so ? :)
    <braunr> it's the contrary actually
    <damo22> VFS -> IPC -> scheduler -> device drivers -> hardware
    <damo22> that is monolithic
    <braunr> well, it's not really meaningful
    <braunr> and i'd say the same applies for a microkernel system
    <damo22> if the application can talk directly to hardware through the
      kernel its almost like plugging directly into the hardware
    <braunr> you never talk directly to hardware
    <braunr> you talk to servers instead of the kernel
    <damo22> ah
    <braunr> consider monolithic kernel systems like systems with one big
      server
    <braunr> the kernel
    <braunr> whereas a multiserver system is a kernel and many servers
    <braunr> you still need the VFS to identify your service (and thus your
      server)
    <braunr> you need much more IPC, since system calls are "replaced" with RPC
    <braunr> the scheduler is basically the same
    <damo22> okay
    <braunr> device drivers are similar too, except they run in thread context
      (which is usually a bit heavier)
    <damo22> but you can do cool things like report when an interrupt line is
      blocked
    <braunr> and there are many context switches between all that
    <braunr> you can do all that in a monolithic kernel too, and faster
    <braunr> but it's far more elegant, and (when well done) easy to do on a
      microkernel based system
    <damo22> yes
    <damo22> i like elegant, makes coding easier if you know the basics
    <braunr> there are only two major differences between a monolilthic kernel
      and a multiserver microkernel system
    * damo22 listens
    <braunr> 1/ independence of location (your resources could be anywhere)
    <braunr> 2/ separation of address spaces (your servers have their own
      addresses)
    <damo22> wow
    <braunr> these both imply additional layers of indirection, making the
      system as a whole slower
    <damo22> but it would be far more secure though i suspect
    <braunr> yes
    <braunr> and reliable
    <braunr> that's why systems like qnx were usually adopted for critical
      tasks
    <damo22> security and reliability are very important, i would switch to the
      hurd if it supported all the hardware i use 
    <braunr> so would i :)
    <braunr> but performance matters too
    <damo22> not to me
    <braunr> it should :p
    <braunr> it really does matter a lot in practice
    <damo22> i mean, a 2x slowdown compared to linux would not affect me
    <damo22> if it had all the benefits we mentioned above
    <braunr> but the hurd is really slow for other reasons than its additional
      layers of indrection unfortunately
    <damo22> is it because of lack of optimisation in the core code?
    <braunr> we're working on these issues, but it's not easy and takes a lot
      of time :p
    <damo22> like you said
    <braunr> yes
    <braunr> and also because of some fundamental design choices related to the
      microkernel back in the 80s
    <damo22> what about the darwin system
    <damo22> it uses a mach kernel?
    <braunr> yes
    <damo22> what is stopping someone taking the MIT code from darwin and
      creating a monster free OS
    <braunr> what for ?
    <damo22> because it already has hardware support
    <damo22> and a mach kernel
    <braunr> in kernel drivers ?
    <damo22> it has kernel extensions
    <damo22> you can do things like kextload module
    <braunr> first, being a mach kernel doesn't make it compatible or even
      easily usable with the hurd, the interfaces have evolved independantly
    <braunr> and second, we really do want more stuff out of the kernel
    <braunr> drivers in particular
    <damo22> may i ask why you are very keen to have drivers out of kernel?
    <braunr> for the same reason we want other system services out of the
      kernel
    <braunr> security, reliability, etc..
    <braunr> ease of debugging
    <braunr> the ability to restart drivers separately, without restarting the
      kernel
    <damo22> i see


# IRC, freenode, #hurd, 2012-09-13

{{$news/2011-q2#phoronix-3}}.

    <braunr> the phoronix benchmarks don't actually test the operating system
      ..
    <hroi_> braunr: well, at least it tests its ability to run programs for
      those particular tasks
    <braunr> exactly, it tests how programs that don't make much use of the
      operating system run
    <braunr> well yes, we can run programs :)
    <pinotree> those are just cpu-taking tasks
    <hroi_> ok
    <pinotree> if you do a benchmark with also i/o, you can see how it is
      (quite) slower on hurd
    <hroi_> perhaps they should have run 10 of those programs in parallel, that
      would test the kernel multitasking I suppose
    <braunr> not even I/O, simply system calls
    <braunr> no, multitasking is ok on the hurd
    <braunr> and it's very similar to what is done on other systems, which
      hasn't changed much for a long time
    <braunr> (except for multiprocessor)
    <braunr> true OS benchmarks measure system calls
    <hroi_> ok, so Im sensing the view that the actual OS kernel architecture
      dont really make that much difference, good software does
    <braunr> not at all
    <braunr> i'm only saying that the phoronix benchmark results are useless
    <braunr> because they didn't measure the right thing
    <hroi_> ok


# Optimizing Data Structure Layout

## IRC, freenode, #hurd, 2014-01-02

    <braunr> teythoon_: wow, digging into the vm code :)
    <teythoon_> i discovered pahole and gnumach was a tempting target :)
    <braunr> never heard of pahole :/
    <teythoon_> it's nice
    <teythoon_> braunr: try pahole -C kmem_cache /boot/gnumach
    <teythoon_> on linux that is. ...
    <braunr> ok
    <teythoon_> braunr: http://paste.debian.net/73864/
    <braunr> very nice


## IRC, freenode, #hurd, 2014-01-03

    <braunr> teythoon: pahole is a very handy tool :)
    <teythoon> yes
    <teythoon> i especially like how general it is