4 ms·

If a process was swapped out and a fault fails to bring a page back due to an IO error, you can at least catch (I think) SIGBUS. But this just reinforces the po

by _wmd 7y ago

If a process was swapped out and a fault fails to bring a page back due to an IO error, you can at least catch (I think) SIGBUS. But this just reinforces the point: nobody really understands virtual memory, even people like us that think they do

klodolph 7y ago
Okay, but I'd say the correct thing to do is let SIGBUS kill the process. You can at least expect to handle SIGBUS for a file you've mapped.
tntn 7y ago
So should we extend your conclusion above to the following? "There is probably enough evidence in this thread to use it as a reference for why typical apps should avoid virtual memory whenever possible -- it's clear almost nobody fully understands it" I'd suggest that is ludicrous, and for the same reason your original conclusion is also excessive.
- _wmd 7y ago
  It is not constructive to form a sweeping generalization from a statement and then claim the sweeping generalization is ludicrous, implying the original statement is ludicrous. :) My first comment was in reply to one claiming anonymous memory did not have the same problems as file-backed memory, indicating the parent did not understand they are the same thing. The subsequent reply was to another comment continuing to claim anonymous memory was somehow safer, both instances supporting the notion that most people in this thread don't seem to understand mmap at all. What we're examining is a powerful (and consequently hazardous) OS feature that often provides only marginal performance improvement, yet introduces many exotic error paths into a program that have their own exotic problems (memory access in thread A can raise SEGV in thread B, async-signal safety), that 7 hours' commenting has not been sufficient to fully capture. This thread is full of upvoted miscomprehension, bad advice (spawn a child to deal with SEGV!?), obviously incorrect solutions (signalfd), and yet still manages to completely omit some critical characteristics of mmap, for example, that faults take a VM-global semaphore -- mmap can easily destroy multithreaded app performance in a way read() is immune to, because nobody expects file IO in one thread to cause malloc() latency in another. If this isn't evidence for "avoid this feature wherever possible", I really don't know what is.
  - tntn 7y ago
    As if "avoid virtual memory" is substantially more of a "sweeping generalization" than "avoid mmap." If you apply the same reasoning that you've used to conclude that everyone should avoid mmap, than you are led directly to the conclusion that everyone should avoid virtual memory. The "same problems" that you are pointing out are possible with anonymous memory aren't unique to memory you get directly from mmap, they also apply to all memory, period. mmap'ed anonymous memory might have th same problems as file-backed memory, but those are the same problems that .text and .bss have. The "powerful (and consequently hazardous) OS feature" here isn't mmap, it's literally virtual memory. At the moment you concede that memory may be backed by something besides physical memory at any point in time, you get the possibility of all those "exotic error paths."
    - klodolph 7y ago
      This is spot on. The exotic error paths are always there, but you don't always need to handle them. You can push some error handling to other systems, such as clients and supervisors / orchestrators. The reason why mmap with files is tractable is because we have a good error handling strategy (remap with zeroes, mark error) and we have a few understandable reasons why we might expect the error (IO errors, lost media/network failures, or even truncate). In general the problem that IO is done outside of direct syscalls like write() can be difficult even when you're not using mmap, like when Postgres was losing errors when calling fsync. But when you have an IO error in your swap file, go ahead, eat the SIGBUS and die. This is fine.
    - _wmd 7y ago
      Let's try and simplify things here: the post we're both currently commenting on relates to doing file IO via nmap. Of course 'avoiding all use of virtual memory' is ridiculous, but nobody except you is suggesting that, and you continue to suggest it even after a long reply. The "powerful (and consequently hazardous) OS feature" here is using mmap for general file IO, it: - introduces resources leaks many developers can't profile - introduces VM bottlenecks 99% of developers can't profile - introduces random segfaults delivered to arbitrary threads in the running process, leading to crashes many developers can't diagnose - the mmap() interface itself is fundamentally unsafe in that it allows partially overwriting random bits of VM (MAP_FIXED) with file views, and worse still, allows those mappings to be read-write Once again, nobody has ever suggested avoiding virtual memory except you -- once again, that is impossible in a modern environment, but it is more than possible, and ultimately incredibly sensible, not to mention entirely on topic with regards to this thread and the article it is attached to, to suggest avoiding use of mmap for general file IO

by _wmd 7y ago

klodolph 7y ago

Okay, but I'd say the correct thing to do is let SIGBUS kill the process. You can at least expect to handle SIGBUS for a file you've mapped.

tntn 7y ago

So should we extend your conclusion above to the following? "There is probably enough evidence in this thread to use it as a reference for why typical apps should avoid virtual memory whenever possible -- it's clear almost nobody fully understands it" I'd suggest that is ludicrous, and for the same reason your original conclusion is also excessive.

_wmd 7y ago
It is not constructive to form a sweeping generalization from a statement and then claim the sweeping generalization is ludicrous, implying the original statement is ludicrous. :) My first comment was in reply to one claiming anonymous memory did not have the same problems as file-backed memory, indicating the parent did not understand they are the same thing. The subsequent reply was to another comment continuing to claim anonymous memory was somehow safer, both instances supporting the notion that most people in this thread don't seem to understand mmap at all. What we're examining is a powerful (and consequently hazardous) OS feature that often provides only marginal performance improvement, yet introduces many exotic error paths into a program that have their own exotic problems (memory access in thread A can raise SEGV in thread B, async-signal safety), that 7 hours' commenting has not been sufficient to fully capture. This thread is full of upvoted miscomprehension, bad advice (spawn a child to deal with SEGV!?), obviously incorrect solutions (signalfd), and yet still manages to completely omit some critical characteristics of mmap, for example, that faults take a VM-global semaphore -- mmap can easily destroy multithreaded app performance in a way read() is immune to, because nobody expects file IO in one thread to cause malloc() latency in another. If this isn't evidence for "avoid this feature wherever possible", I really don't know what is.
- tntn 7y ago
  As if "avoid virtual memory" is substantially more of a "sweeping generalization" than "avoid mmap." If you apply the same reasoning that you've used to conclude that everyone should avoid mmap, than you are led directly to the conclusion that everyone should avoid virtual memory. The "same problems" that you are pointing out are possible with anonymous memory aren't unique to memory you get directly from mmap, they also apply to all memory, period. mmap'ed anonymous memory might have th same problems as file-backed memory, but those are the same problems that .text and .bss have. The "powerful (and consequently hazardous) OS feature" here isn't mmap, it's literally virtual memory. At the moment you concede that memory may be backed by something besides physical memory at any point in time, you get the possibility of all those "exotic error paths."
  - klodolph 7y ago
    This is spot on. The exotic error paths are always there, but you don't always need to handle them. You can push some error handling to other systems, such as clients and supervisors / orchestrators. The reason why mmap with files is tractable is because we have a good error handling strategy (remap with zeroes, mark error) and we have a few understandable reasons why we might expect the error (IO errors, lost media/network failures, or even truncate). In general the problem that IO is done outside of direct syscalls like write() can be difficult even when you're not using mmap, like when Postgres was losing errors when calling fsync. But when you have an IO error in your swap file, go ahead, eat the SIGBUS and die. This is fine.
  - _wmd 7y ago
    Let's try and simplify things here: the post we're both currently commenting on relates to doing file IO via nmap. Of course 'avoiding all use of virtual memory' is ridiculous, but nobody except you is suggesting that, and you continue to suggest it even after a long reply. The "powerful (and consequently hazardous) OS feature" here is using mmap for general file IO, it: - introduces resources leaks many developers can't profile - introduces VM bottlenecks 99% of developers can't profile - introduces random segfaults delivered to arbitrary threads in the running process, leading to crashes many developers can't diagnose - the mmap() interface itself is fundamentally unsafe in that it allows partially overwriting random bits of VM (MAP_FIXED) with file views, and worse still, allows those mappings to be read-write Once again, nobody has ever suggested avoiding virtual memory except you -- once again, that is impossible in a modern environment, but it is more than possible, and ultimately incredibly sensible, not to mention entirely on topic with regards to this thread and the article it is attached to, to suggest avoiding use of mmap for general file IO