# Reading from socket causes massive amounts of memory allocation
**URL:** https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206
**Category:** Performance
**Tags:** question
**Created:** [October 16, 2025, 1:42pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206 "2025-10-16T13:42:30Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 16, 2025, 1:42pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/1 "2025-10-16T13:42:30Z")
Reading from network sockets appears to cause massive amounts of memory allocation.
Does anyone know why? Is this a bug?
```julia-auto
raw_data = Vector{UInt8}(undef, message_length)
read!(socket, raw_data)
```
What I find strange is that the object into which data is written is preallocated with a known length. So there shouldn’t be any allocation taking place here.
What I see is that the number of allocations is approximately the same as the number of elements.
It may be difficult to reproduce this. I can’t provide the original code in its entirety. I have been able to reproduce it by writing small chunks of data to a socket, while the reading end is waiting for a much larger block of memory to complete.
Good numbers to use are an array of 1 million integers.
```julia-auto
using Sockets
sock = Sockets.connect(IPv4("127.0.0.1"), 22222)
v = Vector{Int64}(undef, 1000000)
read!(sock, v)
```
Sending side:
```julia-auto
using Sockets
server = Sockets.listen(IPv4("127.0.0.1"), 22222)
socket = accept(server)
for i in 1:1000
v = Vector{Int64}(undef, 1000)
write(socket, v)
end
```
**Set the receiving side to run first, then trigger the sending side.**
I think that should be enough to reproduce a similar behaviour.
As far as I am aware, I am not doing something “weird” here. (The documentation on sockets is pretty minimal, so it’s hard to know.)
Perhaps this is just not an idiomatic way to use sockets, and that is the cause of the problem. I have no idea.
I am using `--track-allocation=user` to track the memory allocation. Here is one line from one of my `.mem` files.
```julia-auto
25530368 raw_data = Vector{UInt8}(undef, msglen - bytes_already_read)
```
That’s 25 million allocations to read ~ 25 MB of data.
Just some additional tests. This is from `julia --track-allocation=user`, calling `Profile.clear_malloc_data()` after running each function once to trigger the JIT, and then calling each function again to obtain measurements.
```julia-auto
- function example()
0 v = Vector{Int64}(undef, 1000000)
0 return v
- end
-
- function example2(socket)
8000072 v = Vector{Int64}(undef, 1000000)
0 read!(socket, v)
0 return v
- end
```
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [October 16, 2025, 5:50pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/2 "2025-10-16T17:50:34Z")
> [@world-peace](#):
>
> I am using `--track-allocation=user` to track the memory allocation. Here is one line from one of my `.mem` files.
>
> ```julia-auto
> 25530368 raw_data = Vector{UInt8}(undef, msglen - bytes_already_read)
>
> ```
>
> That’s 25 million allocations to read ~ 25 MB of data.
Isn’t that the number of bytes allocated?
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 17, 2025, 12:32pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/3 "2025-10-17T12:32:26Z")
> [@sgaure](#):
>
> Isn’t that the number of bytes allocated?
Is it? I don’t know. I would have thought it was more sensible to track the number of allocations and not the quantity of data allocated.
Allocating 25 million arrays of length 1 byte will have a significant impact on performance. Allocating a single array of 25 million bytes will have close to no impact on performance.
---
### Author:  [@tt1234567](https://discourse.julialang.org/u/tt1234567)
#### Post date: [October 17, 2025, 1:11pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/4 "2025-10-17T13:11:49Z")
I’m not sure it’s the same problem, but some time ago I tried without success to find a function in `Sockets` to read UDP packets which didn’t allocate each time it received data. All the receive functions seem to allocate a new array for each set of data received. See [How to pass a buffer to a C callback function?](https://discourse.julialang.org/t/how-to-pass-a-buffer-to-a-c-callback-function/87716) . I did manage to hack a `recv_into!` function but in the end I wrote the data access in C++ and called it via `CxxWrap`.
I did try creating a feature request [https://github.com/issues/created?issue=JuliaLang|julia|57029](https://github.com/issues/created?issue=JuliaLang%7Cjulia%7C57029)
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 17, 2025, 1:24pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/5 "2025-10-17T13:24:44Z")
The reason I originally asked about this is because I was seeing slow GC times when repeatedly calling a function which reads data from a socket.
I thought the problem was obvious - allocating 25 M elements on the heap when reading that number of bytes from the socket - assuming it was a bug.
However, if the output of the `--trace-allocation` is **bytes** and not **number of allocations** , then there is no bug and the GC is just extremely slow.
eg: ~ 50% of the program runtime is just spent doing GC. This is crazy.
---
### Author:  [@tt1234567](https://discourse.julialang.org/u/tt1234567)
#### Post date: [October 17, 2025, 1:44pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/6 "2025-10-17T13:44:10Z")
> [@world-peace](#):
>
> The reason I originally asked about this is because I was seeing slow GC times when repeatedly calling a function which reads data from a socket.
In my case I tried to avoid allocations as GC temporarily halted all the threads and I lost data. I don’t know much about UDP/TCP comms but allocating on each read seems unnecessary to me.
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 17, 2025, 2:15pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/7 "2025-10-17T14:15:03Z")
> [@tt1234567](#):
>
> allocating on each read seems unnecessary to me
It is certainly unnecessary in the context of
```julia-auto
read!(socket, buffer)
```
because this (should - as far as I know) overwrite the contents of `buffer` with the data from `socket` until enough bytes have been read to fill `buffer`.
---
### Author:  [@tt1234567](https://discourse.julialang.org/u/tt1234567)
#### Post date: [October 17, 2025, 2:32pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/8 "2025-10-17T14:32:18Z")
> [@world-peace](#):
>
> It is certainly unnecessary in the context of
>
> ```julia-auto
> read!(socket, buffer)
>
> ```
>
> because this (should - as far as I know) overwrite the contents of `buffer` with the data from `socket` until enough bytes have been read to fill `buffer`.
You’d think so, but `read!` isn’t defined in `Sockets`. This is well beyond my knowledge, but `Base.read!` seems to use `unsafe_read` and the code for that has the comment “It is recommended that subtypes `T<:IO` override the following method signature
to provide more efficient implementations:” `Sockets` doesn’t seem to provide any such override. Maybe there’s something missing from `Sockets` that I don’t really understand?
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [October 17, 2025, 2:50pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/9 "2025-10-17T14:50:11Z")
I get only a few allocations:
The sender:
```julia-auto
using Sockets
server = Sockets.listen(IPv4("127.0.0.1"), 22222)
socket = accept(server)
v = Vector{Int64}(undef, 1000)
for i in 1:1000
write(socket, v)
end
```
The receiver:
```julia
using Sockets
sock = Sockets.connect(IPv4("127.0.0.1"), 22222)
v = Vector{Int64}(undef, 1000000)
julia> @time read!(sock, v);
0.001464 seconds (9 allocations: 224 bytes)
```
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 20, 2025, 10:31am UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/10 "2025-10-20T10:31:35Z")
The output of this doesn’t make sense to me.
9 allocations, and 224 bytes allocated? To read an array of 8 MB ?
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [October 20, 2025, 10:45am UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/11 "2025-10-20T10:45:56Z")
> [@world-peace](#):
>
> 9 allocations, and 224 bytes allocated? To read an array of 8 MB ?
The `@time read!(sock, v)` reports allocations for the `read!`. The allocation of the vector is in the line before: `v = Vector{Int64}(undef, 1000000)`.
Why `read!` allocates at all, I don’t know.
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 20, 2025, 1:24pm UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/12 "2025-10-20T13:24:19Z")
Ok thanks. Very strange.
---
### Author:  [@world-peace](https://discourse.julialang.org/u/world-peace)
#### Post date: [October 23, 2025, 9:23am UTC](https://discourse.julialang.org/t/reading-from-socket-causes-massive-amounts-of-memory-allocation/133206/13 "2025-10-23T09:23:21Z")
> [@sgaure](#):
>
> I get only a few allocations:
This is the result I see:
```julia-auto
julia> @time read!(sock, v)
0.038267 seconds (162 allocations: 2.609 KiB)
```
Why would my result be significantly different to yours?