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It is not constructive to form a sweeping generalization from a statement and then claim the sweeping generalization is ludicrous, implying the original stateme

by _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
  - klodolph 7y ago
    - What resource leaks are introduced by mmap? - Nobody ever said mmap was always faster than the alternative. If you care about performance then you should do whole-application performance testing with and without features enabled (like mmap IO). This is not unusual, there are plenty of aspects of performance that are counter-intuitive, where speeding up one part of your program causes a seemingly unrelated part of your part of your program to slow down. - The signals are SIGBUS, and they can be intercepted, mapped with zeroes, and the errors can be propagated back to your app code later. This is not trivial but neither is it outrageous. - You can overwrite arbitrary memory with read(), too, you just have to pass it a pointer to something you want to overwrite. mmap() is not any less safe. Recall that typical use of MAP_FIXED is so you can overwrite an existing mmap() region with something else, not so you can nuke random parts of your address space. Keep in mind that you are, if nothing else, an indirect user of mmap(). The question is whether using mmap() directly is advantageous for your applicaiton. "Yes" is not an unreasonable nor outrageous answer for some applications.

by _wmd 7y ago

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
- klodolph 7y ago
  - What resource leaks are introduced by mmap? - Nobody ever said mmap was always faster than the alternative. If you care about performance then you should do whole-application performance testing with and without features enabled (like mmap IO). This is not unusual, there are plenty of aspects of performance that are counter-intuitive, where speeding up one part of your program causes a seemingly unrelated part of your part of your program to slow down. - The signals are SIGBUS, and they can be intercepted, mapped with zeroes, and the errors can be propagated back to your app code later. This is not trivial but neither is it outrageous. - You can overwrite arbitrary memory with read(), too, you just have to pass it a pointer to something you want to overwrite. mmap() is not any less safe. Recall that typical use of MAP_FIXED is so you can overwrite an existing mmap() region with something else, not so you can nuke random parts of your address space. Keep in mind that you are, if nothing else, an indirect user of mmap(). The question is whether using mmap() directly is advantageous for your applicaiton. "Yes" is not an unreasonable nor outrageous answer for some applications.