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Jean-Francois Dockes
2015-06-15 18:13:00 +02:00
parent f19f790b36
commit bab589c7ee
6 changed files with 268 additions and 131 deletions
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website/idxthreads/Makefile
+7 -2
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@@ -1,2 +1,7 @@
threadingRecoll.html : threadingRecoll.txt nothreads.png
asciidoc threadingRecoll.txt
.SUFFIXES: .txt .html
.txt.html:
asciidoc $<
all: threadingRecoll.html forkingRecoll.html xapDocCopyCrash.html
website/idxthreads/forkingRecoll.txt
+73 -46
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@@ -19,39 +19,40 @@ many texts which address the subject. While researching, though, I found
out that not so many were accurate and that a lot of questions were left as
an exercise to the reader.
This document will list the references I found reliable and interesting and
describe the solution chosen along the other possible approaches.
== Issues with fork
The traditional way for a Unix process to start another is the
fork()/exec() system call pair. The initial fork() duplicates the address
space and resources (open files etc.) of the first process, then duplicates
the thread of execution, ending up with 2 mostly identical processes.
exec() then replaces part of the newly executing process with an address space
initialized from an executable file, inheriting some of the old assets
+fork()+/+exec()+ system call pair.
+fork()+ duplicates the process address space and resources (open files
etc.), then duplicates the thread of execution, ending up with 2 mostly
identical processes.
+exec()+ then replaces part of the newly executing process with an address
space initialized from an executable file, inheriting some of the resources
under various conditions.
As processes became bigger the copying-before-discard operation wasted
As processes became bigger the copy-before-discard operation wasted
significant resources, and was optimized using two methods (at very
different points in time):
- The first approach was to supplement fork() with the vfork() call, which
- The first approach was to supplement +fork()+ with the +vfork()+ call, which
is similar but does not duplicate the address space: the new process
thread executes in the old address space. The old thread is blocked
until the new one calls exec() and frees up access to the memory
until the new one calls +exec()+ and frees up access to the memory
space. Any modification performed by the child thread persists when
the old one resumes.
- The more modern approach, which cohexists with vfork(), was to replace
- The more modern approach, which cohexists with +vfork()+, was to replace
the full duplication of the memory space with duplication of the page
descriptors only. The pages in the new process are marked copy-on-write
so that the new process has write access to its memory without
disturbing its parent. The problem with this approach is that the
operation can still be a significant resource consumer for big processes
mapping a lot of memory. Many processes can fall in this category not
because they have huge data segments, but just because they are linked
to many shared libraries.
disturbing its parent. This approach was supposed to make +vfork()+
obsolete, but the operation can still be a significant resource consumer
for big processes mapping a lot of memory, so that +vfork()+ is still
around. Programs can have big memory spaces not only because they have
huge data segments (rare), but just because they are linked to many
shared libraries (more common).
NOTE: Orders of magnitude: a *recollindex* process will easily grow into a
few hundred of megabytes of virtual space. It executes the small and
@@ -60,7 +61,7 @@ indexing multiple such files, *recollindex* can spend '60% of its CPU time'
doing `fork()`/`exec()` housekeeping instead of useful work (this is on Linux,
where `fork()` uses copy-on-write).
Apart from the performance cost, another issue with fork() is that a big
Apart from the performance cost, another issue with +fork()+ is that a big
process can fail executing a small command because of the temporary need to
allocate twice its address space. This is a much discussed subject which we
will leave aside because it generally does not concern *recollindex*, which
@@ -68,16 +69,16 @@ in typical conditions uses a small portion of the machine virtual memory,
so that a temporary doubling is not an issue.
The Recoll indexer is multithreaded, which may introduce other issues. Here
is what happens to threads during the fork()/exec() interval:
is what happens to threads during the +fork()+/+exec()+ interval:
- fork():
- +fork()+:
* The parent process threads all go on their merry way.
* The child process is created with only one thread active, duplicated
from the one which called fork()
- vfork()
* The parent process thread calling vfork() is suspended, the others
from the one which called +fork()+
- +vfork()+
* The parent process thread calling +vfork()+ is suspended, the others
are unaffected.
* The child is created with only one thread, as for fork().
* The child is created with only one thread, as for +fork()+.
This thread shares the memory space with the parent ones, without
having any means to synchronize with them (pthread locks are not
supposed to work across processes): caution needed !
@@ -92,14 +93,14 @@ performed in the child (if no cleanup is performed, pipes may remain open
at both ends which will prevents seeing EOFs etc.). Thanks to StackExchange
user Celada for explaining this to me.
For multithreaded programs, both fork() and vfork() introduce possibilities
For multithreaded programs, both +fork()+ and +vfork()+ introduce possibilities
of deadlock, because the resources held by a non-forking thread in the
parent process can't be released in the child because the thread is not
duplicated. This used to happen from time to time in *recollindex* because
of an error logging call performed if the exec() failed after the fork()
of an error logging call performed if the +exec()+ failed after the +fork()+
(e.g. command not found).
With vfork() it is also possible to trigger a deadlock in the parent by
With +vfork()+ it is also possible to trigger a deadlock in the parent by
(inadvertently) modifying data in the child. This could happen just
link:http://www.oracle.com/technetwork/server-storage/solaris10/subprocess-136439.html[because
of dynamic linker operation] (which, seriously, should be considered a
@@ -110,7 +111,7 @@ In general, the state of program data in the child process is a semi-random
snapshot of what it was in the parent, and the official word about what you
can do is that you can only call
link:http://man7.org/linux/man-pages/man7/signal.7.html[async-safe library
functions] between 'fork()' and 'exec()'. These are functions which are
functions] between +fork()+ and +exec()+. These are functions which are
safe to call from a signal handler because they are either reentrant or
can't be interrupted by a signal. A notable missing entry in the list is
`malloc()`.
@@ -120,8 +121,8 @@ another program (but the devil is in the details as demonstrated by the
logging call issue...).
One of the approaches often proposed for working around this mine-field is
to use an auxiliary, small, process to execute any command needed by the
main one. The small process can just use fork() with no performance
to use an auxiliary small process to execute any command needed by the main
one. The small process can just use +fork()+/+exec()+ with no performance
issues. This has the inconvenient of complicating communication a lot if
data needs to be transferred one way or another.
@@ -164,28 +165,54 @@ descriptors bigger than a specified value (closefrom() equivalent). This is
available on Solaris and quite necessary in fact, because we have no way to
be sure that all open descriptors have the CLOEXEC flag set.
12500 small .doc files:
So, no `posix_spawn()` for us (support was implemented inside
*recollindex*, but the code is normally not used).
fork: real 0m46.025s user 0m26.574s sys 0m39.494s
vfork: real 0m18.223s user 0m17.753s sys 0m1.736s
spawn/fork: real 0m45.726s user 0m27.082s sys 0m40.575s
spawn/vfork: real 0m18.915s user 0m18.681s sys 0m3.828s
== The chosen solution
No surprise here, given the implementation of posix_spawn(), it gets the
same times as the fork/vfork options.
The previous version of +recollindex+ used to use +vfork()+ if it was running
a single thread, and +fork()+ if it ran multiple ones.
It is difficult to ignore the 60% reduction in execution time offered by
using 'vfork()'.
After another careful look at the code, I could see few issues with
using +vfork()+ in the multithreaded indexer, so this was committed.
The only change necessary was to get rid on an implementation of the
lacking Linux +closefrom()+ call (used to close all open descriptors above a
given value). The previous Recoll implementation listed the +/proc/self/fd+
directory to look for open descriptors but this was unsafe because of of
possible memory allocations in +opendir()+ etc.
== Test results
.Indexing 12500 small .doc files
[options="header"]
|===============================
|call |real |user |sys
|fork |0m46.025s |0m26.574s |0m39.494s
|vfork |0m18.223s |0m17.753s |0m1.736s
|spawn/fork| 0m45.726s|0m27.082s| 0m40.575s
|spawn/vfork|0m18.915s|0m18.681s|0m3.828s
|recoll 1.18|1m47.589s|0m21.537s|0m29.458s
|================================
No surprise here, given the implementation of +posix_spawn()+, it gets the
same times as the +fork()+/+vfork()+ options.
The tests were performed on an Intel Core i5 750 (4 cores, 4 threads).
The last line is just for the fun: *recollindex* 1.18 (single-threaded)
needed almost 6 times as long to process the same files...
It would be painful to play it safe and discard the 60% reduction in
execution time offered by using +vfork()+.
To this day, no problems were discovered, but, still crossing fingers...
////
Objections to vfork:
ld.so locks
sigaction locks
https://bugzilla.redhat.com/show_bug.cgi?id=193631
Is Linux vfork thread-safe ? Quoting interesting comments from Solaris
implementation:
No answer to the issues cited though.
implementation: No answer to the issues cited though.
https://sourceware.org/bugzilla/show_bug.cgi?id=378
Use vfork() in posix_spawn()
////