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Re: Debugging Nginx Memory Spikes on Production Servers

Dmitry Volyntsev
September 21, 2023 06:02PM
On 9/21/23 6:50 AM, Lance Dockins wrote:

Hi Lance,

See my comments below.

> Thanky you, Dmitry.
>
> One question before I describe what we are doing with NJS.  I did read
> about the VM handling process before switching from Lua to NJS and it
> sounded very practical but my current understanding is that there
> could be multiple VM’s instantiated for a single request.  A js_set,
> js_content, and js_header_filter directive that applies to a single
> request, for example, would instantiate 3 VMs.  And were you to need
> to set multiple variables with js_set, then keep adding to that # of VMs.
>
>
This is not correct. For js_set, js_content and js_header_filter there
is only a single VM.
The internalRedirect() is the exception, because a VM does not survive
it, but the previous VMs will not be freed until current request is
finished. BTW, a VM instance itself is pretty small in size (~2kb) so it
should not be a problem if you have a reasonable number of redirects.


>
> My original understanding of that was that those VMs would be
> destroyed once they exited so even if you had multiple VMs
> instantiated per request, the memory impact would not be cumulative in
> a single request.  Is that understanding correct?  Or are you saying
> that each VM accumulates more and more memory until the entire request
> completes?
>
> As far as how we’re using NJS, we’re mostly using it for header
> filters, internal redirection, and access control.  So there really
> shouldn’t be a threat to memory in most instances unless we’re not
> just dealing with a single request memory leak inside of a VM but also
> a memory leak that involves every VM that NJS instantiates just
> accumulating memory until the request completes.
>
> Right now, my working theory about what is most likely to be creating
> the memory spikes has to do with POST body analysis.  Unfortunately,
> some of the requests that I have to deal with are POSTs that have to
> either be denied access or routed differently depending on the
> contents of the POST body.  Unfortunately, these same routes can vary
> in the size of the POST body and I have no control over how any of
> that works because the way it works is controlled by third parties.
>  One of those third parties has significant market share on the
> internet so we can’t really avoid dealing with it.
>
> In any case, before we switched to NJS, we were using Lua to do the
> same things and that gave us the advantage of doing both memory
> cleanup if needed and also doing easy analysis of POST body args.  I
> was able to do this sort of thing with Lua before:
> local post_args, post_err = ngx.req.get_post_args()
> if post_args.arg_name = something then
>
> But in NJS, there’s no such POST body utility so I had to write my
> own.  The code that I use to parse out the POST body works for both
> URL encoded POST bodies and multipart POST bodies, but it has to read
> the entire POST into a variable before I can use it.  For small POSTs,
> that’s not a problem.  For larger POSTs that contain a big attachment,
> it would be.  Ultimately, I only care about the string key/value pairs
> for my purposes (not file attachments) so I was hoping to discard
> attachment data while parsing the body.
>
>
>
Thank you for the feedback, I will add it as to a future feature list.

>  I think that that is actually how Lua’s version of this works too.
>  So my next thought was that I could use a Buffer and rs.readSync to
> read the POST body in buffer frames to keep memory minimal so that I
> could could discard the any file attachments from the POST body and
> just evaluate the key/value data that uses simple strings.  But from
> what you’re saying, it sounds like there’s basically no difference
> between fs.readSync w/ a Buffer and rs.readFileSync in terms of actual
> memory use. So either way, with a large POST body, you’d be
> steamrolling the memory use in a single Nginx worker thread. When I
> had to deal with stuff like this in Lua, I’d just run collectgarbage()
> to clean up memory and it seemed to work fine.  But then I also wasn’t
> having to parse out the POST body myself in Lua either.
>
> It’s possible that something else is going on other than that.
>  qs.parse seems like it could get us into some trouble if the
> query_string that was passed was unusuall long too from what you’re
> saying about how memory is handled.
>
>
for qs.parse() there is a limit for a number of arguments, which you can
specify.

>
> None of the situations that I’m handling are for long running
> requests.  They’re all designed for very fast requests that come into
> the servers that I manage on a constant basis.
>
> If you can shed some light on the way that VM’s and their memory are
> handled per my question above and any insights into what to do about
> this type of situation, that would help a lot.  I don’t know if there
> are any plans to offer a POST body parsing feature in NJS for those
> that need to evalute POST body data like how Lua did it, but if there
> was some way to be able to do that at the Nginx layer instead of at
> the NJS layer, it seems like that could be a lot more sensitive to
> memory use.  Right now, if my understanding is correct, the only
> option that I’d even have would be to just stop doing POST body
> handling if the POST body is above a certain total size.  I guess if
> there was some way to forcibly free memory, that would help too.  But
> I don’t think that that is as common of a problem as having to deal
> with very large query strings that some third party appends to a URL
> (probably maliciously) and/or a very large file upload attached to a
> multipart POST.  So the only concern that I’d have about memory in a
> situation where I don’t have to worry about memory when parsing a
> larger file woudl be if multiple js_sets and such would just keep
> spawning VMs and accumulating memory during a single request.
>
> Any thoughts?
>
> —
> Lance Dockins
>
>
> On Thursday, Sep 21, 2023 at 1:45 AM, Dmitry Volyntsev
> <xeioex@nginx.com> wrote:
>
> On 20.09.2023 20:37, Lance Dockins wrote:
>> So I guess my question at the moment is whether endless memory use
>> growth being reported by njs.memoryStats.size after file writes is
>> some sort of false positive tied to quirks in how memory use is
>> being
>> reported or whether this is indicative of a memory leak?  Any
>> insight
>> would be appreicated.
>
> Hi Lance,
> The reason njs.memoryStats.size keeps growing is because NJS uses
> arena
> memory allocator linked to a current request and a new object
> representing memoryStats structure is returned every time
> njs.memoryStats is accessed. Currently NJS does not free most of the
> internal objects and structures until the current request is
> destroyed
> because it is not intended for a long running code.
>
> Regarding the sudden memory spikes, please share some details
> about JS
> code you are using.
> One place to look is to analyze the amount of traffic that goes to
> NJS
> locations and what exactly those location do.
>
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Subject Author Posted

Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 20, 2023 12:56PM

Re: Debugging Nginx Memory Spikes on Production Servers

Maxim Dounin September 20, 2023 03:08PM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 20, 2023 11:38PM

Re: Debugging Nginx Memory Spikes on Production Servers

Dmitry Volyntsev September 21, 2023 02:46AM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 21, 2023 09:52AM

Re: Debugging Nginx Memory Spikes on Production Servers

Dmitry Volyntsev September 21, 2023 06:02PM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 21, 2023 07:42PM

Re: Debugging Nginx Memory Spikes on Production Servers

Dmitry Volyntsev September 21, 2023 08:48PM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 22, 2023 09:36AM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 26, 2023 11:32AM

Re: Debugging Nginx Memory Spikes on Production Servers

Dmitry Volyntsev September 26, 2023 06:52PM

Re: Debugging Nginx Memory Spikes on Production Servers

Lance Dockins September 26, 2023 09:22PM



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